Cutshort logo
Pilani Pixels Digital Services logo
Senior Blockchain Developer
Pilani Pixels Digital Services's logo

Senior Blockchain Developer

Muskan Gupta's profile picture
Posted by Muskan Gupta
4 - 6 yrs
₹15L - ₹24L / yr
Dwarka
Skills
Blockchain
Wallet (Blockchain)
Web3js

Min Exp – 4 years

Location – Dwarka, Delhi

Immediate joiners are preferred


About the company – We are a web development and digital marketing company in Delhi, India. We have been serving our clients globally since 2012. We offer quality assured and revenue driven digital services at affordable rate.


Role summary - We seek an experienced Web3 Developer to join our team and help us build and maintain our Ecosystem based on blockchain. Our platform is built using React for the front end, Node.js, TypeScript, MongoDB for the backend, and Solidity for smart contracts. Someone passionate about blockchain technology and with a strong understanding of the Web3 ecosystem.


Responsibilities:

1. Blockchain Solution Design:

  • Collaborate with stakeholders to understand business requirements and identify areas where blockchain technology can add value.
  • Design and architect blockchain solutions, including selecting appropriate blockchain platforms, consensus mechanisms, data structures, and smart contract frameworks.
  • Develop technical specifications, system architectures, and integration strategies for blockchain-based applications.
  • Ensure blockchain solutions align with industry best practices, security standards, and compliance requirements.

2. Blockchain Development:

  • Lead the development and implementation of blockchain applications, smart contracts, and decentralized protocols.
  • Design and implement APIs and integration points for seamless interaction between blockchain and existing enterprise systems.
  • Conduct thorough testing and debugging to ensure blockchain solutions' performance, security, and reliability.
  • Collaborate with development teams to define blockchain development coding standards and best practices.

3. Security and Privacy:

  • Implement robust security measures, encryption techniques, and access controls to protect blockchain data and transactions.
  • Address privacy concerns and design solutions that ensure the confidentiality of sensitive information while maintaining transparency.
  • Stay up-to-date with the latest security vulnerabilities and recommend strategies to mitigate risks in blockchain implementations.

4. Scalability and Performance Optimization:

  • Optimize blockchain solutions to enhance scalability, transaction throughput, and system performance.
  • Evaluate and implement scaling solutions, such as sharding, sidechains, or layer 2 protocols, to address the limitations of blockchain platforms.
  • Conduct load testing and performance tuning to identify bottlenecks and optimize resource utilization.

5. Blockchain Governance and Consensus:

  • Define governance models and consensus mechanisms suitable for specific use cases and industry requirements.
  • Assess and recommend appropriate consensus algorithms, such as Proof of Work (PoW), Proof of Stake (PoS), or Practical Byzantine Fault Tolerance (PBFT), based on the project's needs.
  • Establish protocols for transaction validation, block verification, and network participation.

6. Research and Innovation:

  • Stay updated with the latest advancements and emerging trends in blockchain technology.
  • Conduct research and experimentation to explore the potential of new blockchain platforms, protocols, and tools.
  • Evaluate and recommend third-party blockchain solutions, frameworks, and libraries to accelerate development processes.

7. Collaboration and Leadership:

  • Collaborate with cross-functional teams, including developers, engineers, product managers, and business stakeholders, to align technical requirements with business goals.
  • Provide technical guidance and mentorship to development teams regarding blockchain technologies, best practices, and coding standards.
  • Act as a subject matter expert, delivering presentations and training sessions on blockchain concepts and solutions.


Qualifications and Skills:


  • Bachelor's or Master's degree in Computer Science, Information Technology, or a related field.
  • Extensive experience in blockchain development and architecture, with a deep understanding of blockchain concepts, protocols, and platforms (such as Ethereum, Hyperledger Fabric, Corda, etc.).
  • Proficiency in programming languages commonly used in blockchain development, such as Solidity, Go, or Java.
  • Strong knowledge of smart contracts, decentralized applications (DApps), and blockchain development frameworks.
  • Familiarity with cryptographic principles, encryption algorithms, and security best practices in the context of blockchain.
  • Experience with cloud platforms and infrastructure (such as AWS, Azure, or Google Cloud) for deploying and managing blockchain networks.
  • Understanding of decentralized storage, identity management, and tokenization concepts.
  • Excellent problem-solving and analytical skills, with the ability to architect complex and scalable blockchain solutions.
  • Strong communication and leadership skills to collaborate effectively with multidisciplinary teams and drive successful project outcomes.
Read more
Users love Cutshort
Read about what our users have to say about finding their next opportunity on Cutshort.
Shubham Vishwakarma's profile image

Shubham Vishwakarma

Full Stack Developer - Averlon
I had an amazing experience. It was a delight getting interviewed via Cutshort. The entire end to end process was amazing. I would like to mention Reshika, she was just amazing wrt guiding me through the process. Thank you team.
Companies hiring on Cutshort
companies logos

About Pilani Pixels Digital Services

Founded :
2020
Type :
Products & Services
Size :
0-20
Stage :
Profitable

About

We are a web development and digital marketing company in Delhi, India. We have been serving our clients globally since 2012. We offer quality assured and revenue driven digital services at affordable rate.
Read more

Company social profiles

instagramlinkedintwitterfacebook

Similar jobs

Flipr
Remote only
4 - 5 yrs
₹15L - ₹18L / yr
Web3js
Blockchain
Ethereum
Solidity
polygon
+1 more

**Role Overview:** We are seeking a skilled Web3 blockchain developer with strong experience in Ethereum and decentralized finance (DeFi). The ideal candidate should have expertise in smart contract development, ERC20 tokens, security best practices, and experience integrating with blockchain payment gateways or APIs related to stablecoins.


**Key Responsibilities:**

* Develop and deploy ERC20 smart contracts, including token minting, redemption, and transfer functionalities on Ethereum.

* Design, review, and optimize smart contracts related to crypto lending, escrow, collateral, and repayments.

* Integrate with Circle’s platform for USDC operations, understanding minting and redeeming processes via its APIs.

* Assess stability mechanisms of USDC, ensuring secure handling of minting, redemption workflows, and transactions.

* Collaborate with backend systems (Node.js / Typescript) to facilitate blockchain interactions, manage transactions, and implement error recovery.

* Implement blockchain security best practices, including transaction signing, API key management, and event validation.

* Explore interoperability between Ethereum and other blockchain networks as needed, considering costs and operational efficiency.

* Work closely with frontend and backend teams to embed blockchain transaction logic, wallet management, and transaction tracking.

* Document smart contract code, API workflows, and integration processes to support compliance and audits.

* Ensure platform adherence to relevant regulatory standards for crypto operations.

* Requirements:

* Proven experience in Ethereum smart contract development (Solidity), with a focus on ERC20 standards.

* Experience with DeFi protocols, escrow, lending, or collateral management smart contracts.

* Familiarity with Circle’s platform APIs, especially for USDC minting and redemption workflows, is a strong advantage.

* Hands-on experience with blockchain libraries such as web3.js, ethers.js, or similar.

* Understanding of blockchain transaction costs, network throughput, and interoperability considerations.

* Strong knowledge of smart contract security, auditing, and safe transaction handling.

* Experience working with backend environments like Node.js or Typescript.

* Basic familiarity with Circle’s wallet APIs, webhook integrations, and transaction validation best practices.

* Excellent coding, debugging, and documentation skills.

* Ability to work independently and collaboratively in a dynamic environment.


**Preferred Skills:**

* Experience across multiple blockchain ecosystems (e.g., Ethereum, Polygon, Solana).

* Prior exposure to Circle’s API systems and USDC management workflows.

* Strong security awareness around wallet management, API key handling, and transaction validation.

* Knowledge of DeFi security audits, formal verification, or related security frameworks.

* Contributions to open-source blockchain projects or experience in blockchain community engagement.

Read more
Blockchian Technology
Blockchian Technology
Agency job
Bengaluru (Bangalore)
4 - 6 yrs
₹8L - ₹14L / yr
Blockchain
Wallet (Blockchain)
Whisper (Blockchain)
Hyperledger
InterPlanetary File System (IPFS)
+5 more
  • 4-6 years of proven experience as a Full stack engineer
  • 2+ years of Blockchain development experience in Hyperledger Fabric, IPFS and development of smart contracts in Golang.
  • Good understanding of multiple Blockchain platforms such as Hyperledger Fabric, Quorum, Ethereum etc
  • Experience in Java, JavaScript Experience in developing and designing RESTful services and APIs
  • Experience in Microservices architecture
  • Sound knowledge on Docker and Kubernetes applications
  • Experience in Banking, Financial and Fintech domain is an added advantage
  • Familiarity with Blockchain protocols is a plus
  • Knowledge of Agile development environment is preferable
  • Ability to work within a fast-paced, collaborative environment and handling multiple projects
  • Customer-first mentality, intellectual curiosity, and hunger
  • Interested in learning new technologies, creative, open-minded and a great team player
  • Bachelors or master’s degree in engineering (preferably Computer Science or Information Technology)
Read more
Diva Global school
at Diva Global school
1 recruiter
Priya Thakur
Posted by Priya Thakur
Jaipur
1 - 3 yrs
₹4.8L - ₹6.8L / yr
Blockchain
Ethereum
Hyperledger

Job Summary 

We are looking for a blockchain developer who will be responsible for designing, implementing, and supporting a distributed blockchain-based network.

 Experience: 2+

Location: Jaipur 


Responsibilities: 

  • Research, design, develop, and test blockchain applications
  • Brainstorm and help evaluate applications, technologies and tools as these continually evolve
  • Maintain and extend current distributed applications, be responsible for its integration and workflows
  • Document new solutions as well as maintaining that of existing ones
  • Develop and maintain artefacts
  • Participate in presales activities when it is required 


Requirements:                       

  • Strong blockchain development experience on network, protocol and application layer
  • Experience working with open-source platforms Hyperledger, Ethereum etc.
  • Understanding the private blockchain network. Desirable.
  • Experience with different PAAS solutions offered by different cloud platforms such as Azure, AWS etc. Desirable.
  • Strong experience with computer networking is desirable including details of network topology and OSI layer protocols.
  • API integration experience with 3rd party applications is desirable
  • Experience with Secure coding practices is a plus
  • Think out of the box and don't give up easily, as blockchain is a fairly complex concept and there isn't much help available in programmers forums.

 

Role:Blockchain Developer

Salary: 4,00,000 - 5,400,000 P.A.

Industry:https://www.naukri.com/it-services-consulting-jobs">IT Services & Consulting

Functional Area:https://www.naukri.com/consulting-jobs">Consulting

Role Category:IT Consulting

Employment Type:Full Time, Permanent

Education

UG:B.Tech/B.E. in Any Specialization

Company Profile

Appinop  technologies Pvt ltd

About Appinop

Innovation begins with like-minded people aiming to transform the world together.

We invite you to become a part of an organization that has been helping clients transform their business by fusing insights, creativity, and technology. At Appinop, we embrace a diverse mix of talented people who come here to stay and do their best work. 

At Appinop, we transform passionate individuals into proficient professionals dedicated to exploring new frontiers in Software development. Here you will get the opportunity to work with technical craftsmen that are pioneer in latest technologies

We promote a culture of equality, learning, collaboration, and creative freedom so that our employees grow.

For more information about our solutions and organization, visit www.Appinop.com, 

 

Company Info:

Corporate Address: Appinop Technologies - 41, Prince Road, Ajmer Rd, Vidhyut Nagar - A, Jaipur, Rajasthan 302021

 

Read more
Teblox Labs
Ritika Rai
Posted by Ritika Rai
Delhi, Noida, Ghaziabad, Bengaluru (Bangalore), Hyderabad, Pune, Mumbai
3 - 15 yrs
₹7L - ₹25L / yr
Blockchain
cryptocurrency

Job Description:

We are seeking a skilled and experienced Blockchain Developer to join our dynamic team. As a Blockchain Developer, you will be responsible for designing, implementing, and maintaining blockchain-based applications and solutions. You will collaborate with cross-functional teams to develop innovative blockchain solutions that meet our business requirements. The ideal candidate should have a strong background in blockchain technology, solid programming skills, and a passion for staying updated with the latest trends and developments in the blockchain space.

Responsibilities:

  1. Design, develop, and implement blockchain solutions based on business requirements.
  2. Collaborate with cross-functional teams to integrate blockchain technology into existing systems and applications.
  3. Research and evaluate blockchain frameworks, platforms, and tools to identify the most suitable solutions for our projects.
  4. Write clean, efficient, and maintainable code for blockchain applications.
  5. Implement smart contracts and decentralized applications (DApps) using Solidity or other relevant programming languages.
  6. Ensure the security, scalability, and performance of blockchain solutions.
  7. Conduct code reviews, testing, and debugging to ensure the quality of deliverables.
  8. Stay updated with the latest developments, standards, and best practices in blockchain technology.
  9. Provide technical guidance and support to team members and stakeholders.
  10. Document technical specifications, architecture, and processes related to blockchain development projects.

Requirements:

  1. Bachelor's or Master's degree in Computer Science, Engineering, or related field.
  2. Proven experience as a Blockchain Developer or related role.
  3. Strong understanding of blockchain technology, cryptocurrencies, and decentralized systems.
  4. Proficiency in programming languages such as Solidity, JavaScript, Python, or C++.
  5. Experience with blockchain platforms such as Ethereum, Hyperledger, Corda, or EOS.
  6. Familiarity with smart contract development and deployment.
  7. Knowledge of cryptographic principles and security protocols.
  8. Experience with blockchain development tools, libraries, and frameworks.
  9. Excellent problem-solving skills and attention to detail.
  10. Strong communication and teamwork skills.
  11. Ability to work effectively in a fast-paced environment and manage multiple tasks simultaneously.
  12. Certification in blockchain development (e.g., Certified Blockchain Developer) is a plus.

Join our team and be part of exciting projects at the forefront of blockchain technology innovation!

Read more
ZeMoSo Technologies
at ZeMoSo Technologies
11 recruiters
HR Team
Posted by HR Team
Remote only
4 - 6 yrs
₹10L - ₹25L / yr
Hyperledger
Blockchain
skill icon.NET
skill iconJava
Linux/Unix
What's this role about?
 
[Note: This is a profile of an experienced Hyperledger Developer (must NOT be very junior)]
 
We are looking for an experienced Senior Blockchain Developer (Hyperledger) with industry experience in designing, solutioning and implementing real-world systems, across various technologies.
 
Candidate should have at least 4+ (and up to 6 years relevant) years of experience with clear responsibility as a Blockchain (Hyperledger) developer and at least 2+ years of experience in designing and building Hyperledger Blockchain-based solutions.
 
(Overall, 4 - 6 years of experience)
 
Here's how you'll contribute:
 
  1. Work as a Senior Developer or Tech Lead for blockchain-based projects.
  2. Lead small development teams (3-5 people).
  3. Work in Agile/Scrum teams.
  4. Deliver quality solutions to customers by implementing relevant tools/ frameworks/ libraries in Hyperledger.
 
Skills required to contribute:
 
Required Skills
 
  1. Experience in the role of a senior developer/tech lead for blockchain and non-blockchain-based projects. (At least 3 to 5 projects)
  2. Hands-on experience with Enterprise Blockchain platforms (particularly Hyperledger ONLY) and a good awareness of others, to evaluate and drive the selection of the right platform for the area of implementation.
  3. Must be well versed with software design and development methodologies for web application development using Java/Java EE/Open Source or Microsoft .NET technologies.
  4. Mandatory experience in implementing Blockchain applications/projects using MEAN/MERN/Java/Go technology stack, Git, Linux/ Ubuntu/ MacOs/ Windows/ Virtual machines.
  5. Understanding of smart contracts and experience in developing smart contracts (Node/ JavaScript/ Go/ Java) for at least 2 live projects.
  6. Knowledge of design considerations, best practices, and standards for designing smart contract-based solutions for decentralized applications.
  7. Understanding of common tools/ frameworks/ libraries used for quality software development and Blockchain implementation, including those for Hyperledger.
  8. Knowledge of common architecture patterns for applications (MVC, SPA), cloud, data storage, security, API/Integration, serverless architectures, etc.
  9. Experience in using (and configuring) CI/CD pipelines (and know-how of DevOps) for blockchain projects.
  10. Experience in leading small development teams (3-5 people) and should have worked in Agile/Scrum teams.
  11. Strong verbal and written communication skills; must be comfortable with client and stakeholder interactions.
  12. Good understanding of IPFS and IPNS protocols.
 
Desirable
 
  1. Interest to learn and apply learning to design and implement blockchain solutions
  2. Record of publication of good quality blogs and papers.
  3. Completion of relevant certifications: Blockchain developer, Java EE / Web Application Developer, Cloud Certification, etc.


About us:

Zemoso Technologies is a Software Product Market Fit Studio that brings silicon valley style rapid prototyping and rapid application builds to Entrepreneurs and Corporate innovation. We offer Innovation as a service and work on ideas from scratch and take it to the Product Market Fit stage using Design Thinking->Lean Execution->Agile Methodology.

We were featured as one of Deloitte Fastest 50 growing tech companies from India thrice (2016, 2018 and 2019). We were also featured in Deloitte Technology Fast 500 Asia Pacific both in 2016 and 2018.

We are located in Hyderabad, India, and Dallas, US. We have recently incorporated another office in Waterloo, Canada.

Our founders have had past successes - founded a decision management company acquired by SAP AG (now part of Hana Big data stack & NetWeaver BPM), early engineering team of Zoho (leading billion $ SaaS player) & some Private Equity experience.


Marquee customers along with some exciting start-ups are part of our clientele.

Read more
EY (Ernst & Young)
Jaipur
15 - 20 yrs
₹25L - ₹28L / yr
Blockchain
Project Management
Hyperledger
Ethereum
R3 Corda

Responsibilities:


  • Collaborate with R&D teams and incorporate blockchain into the development and design of solutions that employ cross-asset frameworks and concepts
  • Leadership abilities in order to provide the team with the correct direction to ensure that the correct solution is implemented with tremendous success.
  • Build performance indicators to assess the performance of the Blockchain.
  • Preparation of a risk and failure analysis and risk prevention plan.
  • Train and advise technical employees on Blockchain tools and frameworks, Blockchain principles, and so forth.


Required Skills:


  • Worked on new technologies and develop performance standards for appraising any new solutions.
  • Extensive knowledge of blockchain architectural concepts and the development of storage solutions.
  • He shall have a solid understanding of Blockchain foundations, consensus algorithms, and smart contracts 
  • Excellent knowledge of blockchain frameworks, for example, Hyperledger Fabric, R3 Corda, and Ethereum
  • Good command over the several consensus algorithms used for the Bitcoin Blockchain, Ethereum, Byzantine fault-tolerant algorithms
  • Strong understanding of cryptography, in order to provide greater security and immutability to the system


Read more
Rejolut
at Rejolut
1 video
2 recruiters
TalentA Team
Posted by TalentA Team
Navi Mumbai, Mumbai
1 - 5 yrs
₹1L - ₹18L / yr
Blockchain
skill iconRust
Smart Contracts
Cryptocurrency
Crypto

We are looking for a Blockchain Developer who has around 2-3 years of experience, and has past experience with working on blockchain-based systems and scenarios. The ideal candidate would be someone who understands the concepts of Blockchain technology and having working experience on Rust and Solana.

  • Desire to work in a startup environment, fun@work culture and able to self-manage.

  • In Depth understanding of at-least two blockchain network like Rust, Substrate, Polkadot Solana, Hedera Hashgraph, Tezos, BlockApps, Algorand, Corda, Lambda, Cargo etc

  • Familiarity with basic cryptography to understand the essentials of blockchain systems.

  • Hands on in Rust

  • Designing and writing well-structured APIs in Node.js and MongoDb to connect to Blockchain infrastructure

  • Staying up to date with new developments and news in integrated chains.

  • Past experience with Wallets or Crypto Exchanges would be a major plus.

  • Experience with third-party libraries and APIs.

  • Experience working in an agile development process.

  • Documenting architecture and guidelines.

  • Be proficient in using version control and continuous integration, with tools such as Gitlab, Git and Jenkins.

  • Write clear, maintainable and testable code that you are proud of.

  • Attend training and education where necessary to up-skill themselves.

  • Experience in writing smart contracts for large-scale production blockchain applications.

  • Cross-team contribution whenever required.

  • Familiarity with basic cryptography.

  • Should have a deep understanding of the blockchain and cryptocurrency space and smart contract systems, specifically having a deep understanding of Rust.

Read more
Vibhathi Labs
at Vibhathi Labs
1 recruiter
Tanya Thakur
Posted by Tanya Thakur
Remote only
4 - 10 yrs
₹30L - ₹50L / yr
Solidity
Ethereum
Truffle (Ethereum framework)
Web3js
Smart Contracts
+2 more

We are looking for a blockchain developer who will be responsible for designing, implementing, and supporting a distributed blockchain-based network. Primary responsibility will be analyzing requirements, designing blockchain technology around a certain business model, and building and launching a blockchain network.

 

Essential Skills :

 

  • Strong software development background

  • Experience working with large codebases

  • Solidity, Web3.js, Ethereum, Truffle, Remix, Smart Contracts, Cryptography, Wallets, Blockchain Fundamentals, DeFi & Banking Domain

  • Experience working with open-source projects

  • Proficiency in one of the following languages: C++, Java, JavaScript 

  • Strong knowledge of common algorithms and data structures

  • Familiarity with basic cryptography

  • Familiarity with P2P networks

  • Strong knowledge of bitcoin-like blockchains / Stable coins

  • Strong knowledge of concurrency and writing efficient and safe multithreaded code

  • Strong knowledge of STL, C++11 

  • Proficiency in the Qt Widgets module 

 

Responsibilities :

 

  • Research, design, develop, and test blockchain technologies

  • Brainstorm and help evaluate applications for new tools and technologies as they continually evolve

  • Maintain and extend current client- and server-side applications responsible for integration and business logic

  • Be involved in the global blockchain community—work on implementing and integrating the latest improvement proposals

  • Document new solutions as well as maintain of existing ones

Read more
Mobile Programming India Pvt Ltd
at Mobile Programming India Pvt Ltd
1 video
17 recruiters
indu saini
Posted by indu saini
Mohali
6 - 9 yrs
₹1L - ₹11L / yr
Blockchain
Cryptography
Ethereum
Smart Contracts
NFT's
designation: Block Chain Developer
Skillset: strong knowledge of  NFT's
Exp: 6+ Years
looking for Freelancer
Read more
asva finance
at asva finance
1 recruiter
Vidya reddy
Posted by Vidya reddy
Remote only
3 - 5 yrs
$5K - $10K / yr
Blockchain
Cryptography
Smart Contracts
Ethereum
polkadot
+2 more

Job description for Blockchain Developer

Responsibilities

• Create architecture features and interfaces by using programming languages
• Research and design blockchain technologies
• Develop and test blockchain technologies – Ethereum and Polkadot
• Protecting the application from bugs and cyber-attacks.
• Working closely with engineers, sales personnel, and other teams in designing and creating innovative solutions.
• Research, create and document existing and new blockchain structures and proposed solutions
• Writing excellent multithreaded codes
• Analysing the needs, resources, and problems of the company and provide necessary
solutions
• Working with open-source projects and codebases
• Keeping up with current blockchain technology and cryptocurrency trends

Requirement skills

• Strong background in software development or engineering with 2-3 years of relevant experience
• Strong conceptual knowledge on DeFi platforms
• Experience with solidity and node.js must
• Knowledge in go or rust
• Strong back-end skills
• Algorithms & data structures
• Basic knowledge of cryptography
• Peer-to-peer or P2P networks
• Qt Widgets module for GUI wallet application.
• Knowledge of current technology like microservice architectures and Docker
containers.
• B.E/B.Tech/BSc in Computer Science, Mathematics or similar field; Master’s degree
is a plus

Salary: Based on industry standards

Read more
Why apply to jobs via Cutshort
people_solving_puzzle
Personalized job matches
Stop wasting time. Get matched with jobs that meet your skills, aspirations and preferences.
people_verifying_people
Verified hiring teams
See actual hiring teams, find common social connections or connect with them directly.
ai_chip
Move faster with AI
We use AI to get you faster responses, recommendations and unmatched user experience.
Did not find a job you were looking for?
icon
Search for relevant jobs from 10000+ companies such as Google, Amazon & Uber actively hiring on Cutshort.
companies logo
companies logo
companies logo
companies logo
companies logo
Get to hear about interesting companies hiring right now
Company logo
Company logo
Company logo
Company logo
Company logo
Linkedin iconFollow Cutshort
Users love Cutshort
Read about what our users have to say about finding their next opportunity on Cutshort.
Shubham Vishwakarma's profile image

Shubham Vishwakarma

Full Stack Developer - Averlon
I had an amazing experience. It was a delight getting interviewed via Cutshort. The entire end to end process was amazing. I would like to mention Reshika, she was just amazing wrt guiding me through the process. Thank you team.
Companies hiring on Cutshort
companies logos