Backend Engineer

Oscar Associates Limited; Greater London; Hybrid; GBP 85,000 - 105,000; Full time

Job summary

Oscar Associates Limited is hiring a Backend Engineer to build scalable SaaS platforms from London. You will design REST and asynchronous services in Python 3.11 with FastAPI, interfacing across Java/Spring boundaries and PostgreSQL persistence.

You’ll ensure tenant isolation, access control and robust testing, while deploying on AWS (EKS/ECS, RDS, S3) and integrating OpenTelemetry/logging.

Qualifications

• At least 5 years of back-end engineering experience in production SaaS or platform systems.
• Experience with multi-tenant or enterprise systems, including isolation, permissions and auditability
• Strong Python skills and experience with FastAPI or comparable frameworks
• Solid SQL and PostgreSQL skills, with the ability to work at Java/Spring service boundaries
• Production AWS experience and confidence with Git, CI/CD and Linux
• Daily use of AI coding tools such as Cursor or Claude Code, with a considered view of where they help
• A degree in engineering, computer science or a related field, or equivalent experience
• We value Java/Spring depth alongside Python, event-driven architecture, queues and workflow engines, financial services or regulated-industry experience, and LLM service integration patterns
• For interview preparation, be ready to discuss an API contract evolved without breaking consumers, a tenant or permission boundary implemented and tested, and a production quality, reliability or cost problem diagnosed and improved

Responsibilities

• Design REST and asynchronous services in Python 3.11 and FastAPI
• Work at Java/Spring boundaries with PostgreSQL and async SQLAlchemy persistence
• Implement tenant isolation, context, role-based access and configuration
• Version and evolve contracts between core and AI services without breaking consumers
• Maintain comprehensive unit and integration tests with pytest and pytest-asyncio, and uphold code review standards
• Use OpenTelemetry traces, structured logs, metrics and actionable alerts to expose model-call latency, failure rates and token costs
• Build validation, schema enforcement and safe-failure behaviour for outputs that may be well-formed yet incorrect
• Apply authentication, authorisation, secrets management, input validation, OWASP awareness and audit trails in everyday engineering
• Deploy and operate services on AWS using EKS/ECS patterns, RDS and S3 through Jenkins quality gates
• Improve queries, caching, asynchronous throughput and resource right-sizing
• Build queueing, rate limits and cost-aware orchestration for model-calling services
• Ship through disciplined CI/CD alongside the India-based delivery organisation
• Create evaluations, visibility and safeguards to catch mistakes early, reduce them over time and prevent recurring failures

Skills

Back-end engineering

Python

PostgreSQL

AWS

CI/CD

Linux

AI tooling

Multi-tenant/enterprise

REST API design

Education

Engineering degree or equivalent

Tools

FastAPI

SQLAlchemy

OpenTelemetry

Jenkins

AWS EKS/ECS

Job description

Salary: £100,000 - 100,000 per year

• At least 5 years of back-end engineering experience in production SaaS or platform systems
• Experience with multi-tenant or enterprise systems, including isolation, permissions and auditability
• Strong Python skills and experience with FastAPI or comparable frameworks
• Solid SQL and PostgreSQL skills, with the ability to work at Java/Spring service boundaries
• Production AWS experience and confidence with Git, CI/CD and Linux
• Daily use of AI coding tools such as Cursor or Claude Code, with a considered view of where they help
• A degree in engineering, computer science or a related field, or equivalent experience
• We value Java/Spring depth alongside Python, event-driven architecture, queues and workflow engines, financial services or regulated-industry experience, and LLM service integration patterns
• For interview preparation, be ready to discuss an API contract evolved without breaking consumers, a tenant or permission boundary implemented and tested, and a production quality, reliability or cost problem diagnosed and improved

Responsibilities:

• Design REST and asynchronous services in Python 3.11 and FastAPI
• Work at Java/Spring boundaries with PostgreSQL and async SQLAlchemy persistence
• Implement tenant isolation, context, role-based access and configuration
• Version and evolve contracts between core and AI services without breaking consumers
• Maintain comprehensive unit and integration tests with pytest and pytest-asyncio, and uphold code review standards
• Use OpenTelemetry traces, structured logs, metrics and actionable alerts to expose model-call latency, failure rates and token costs
• Build validation, schema enforcement and safe-failure behaviour for outputs that may be well-formed yet incorrect
• Apply authentication, authorisation, secrets management, input validation, OWASP awareness and audit trails in everyday engineering
• Deploy and operate services on AWS using EKS/ECS patterns, RDS and S3 through Jenkins quality gates
• Improve queries, caching, asynchronous throughput and resource right-sizing
• Build queueing, rate limits and cost-aware orchestration for model-calling servicesShip through disciplined CI/CD alongside the India-based delivery organisation
• Create evaluations, visibility and safeguards to catch mistakes early, reduce them over time and prevent recurring failures

Technologies:

• AI
• API
• AWS
• asyncio
• Backend
• CI/CD
• Claude Code
• Cursor
• FastAPI
• Git
• Java
• Jenkins
• LLM
• Linux
• OWASP
• OpenTelemetry
• PostgreSQL
• Python
• REST
• SQL
• Spring
• pytest
• Cloud

More:

We are hiring a Backend Engineer for a newly formed software venture backed by an established global investment-services business. You will build the platform behind its product, joining one of nine engineering hires across front-end and back-end and reporting to the Head of Engineering as part of our UK core team. The opportunity is to establish the foundations of an enterprise-grade technology platform aimed at a multi-billion-dollar serviceable market and share in its growth. The work combines pace and accuracy in high-stakes live businesses, where autonomous systems can make mistakes and decisions may need to be made without full consensus. The role is permanent, based in Kings Cross, London, with three days a week in the office; colleagues are split between London and an India-based delivery organisation. Salary is £85,000–£105,000. Visa sponsorship is not available. We aim to conclude the interview process within three weeks, and interview adjustments can be arranged on request.