Backend Engineer

Oscar Technology; Greater London; On-site; GBP 85,000 - 105,000; Full time

Job summary

Oscar Technology is seeking a Backend Engineer in London to design and implement multi-tenant data/workflow services as part of a high-growth platform. You’ll integrate Python AI services with a Java/Spring core and own the API layer in a regulated enterprise context.

You’ll report to the UK core team, work primarily in-office at Kings Cross (3 days per week), and collaborate across a dispersed delivery organisation to ship reliable services with strong auditability.

Qualifications

• 5+ years of backend engineering for production SaaS or platforms.
• Experience with multi-tenant systems, isolation, permissions and auditability.
• Strong Python with FastAPI and PostgreSQL, able to work at Java/Spring boundaries.
• Proficient in AWS, Git, CI/CD and Linux.
• Familiar with AI coding tools and evaluating their value in workflows.

Responsibilities

• Design REST and asynchronous services in Python 3.11+ and FastAPI.
• Work at Java/Spring boundaries with PostgreSQL and async SQLAlchemy.
• Implement tenant isolation, context, role-based access and configuration.
• Maintain CI/CD and collaborate with cross-continental teams.

Skills

Python

FastAPI

PostgreSQL

AWS

CI/CD

Linux

AI tools

Education

Engineering degree or equivalent experience

Job description

Backend Engineer

• Location: Kings Cross, London
• Salary: £85,000-105,000
• Contract: Permanent
• Working pattern: In office 3 days a week

A newly formed software venture backed by an established global investment-services business is hiring a Backend Engineer to build the platform its product runs on. You'll design multi-tenant data and workflow services in Python and FastAPI and own the integration layer between a Java/Spring core and Python AI services. This is one of nine engineering hires across front-end and back-end.

THE ROLE

Investment professionals need APIs and workflows that respect permissions, keep an audit trail and fail safely when an AI result can't be trusted. You'll build the services behind that.

• Report to the Head of Engineering as part of the UK core team.
• Build multi-tenant data and workflow services, plus the integration layer between the Java/Spring core and the Python AI services.
• Ship through disciplined CI/CD alongside the India-based delivery organisation.
• Combine service design with the production constraints of demanding enterprise users.
• Join a nine-person engineering cohort being hired across front-end and back-end.

WHAT YOU'LL BE DOING - SERVICES AND CONTRACTS

• 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.

WHAT YOU'LL BE DOING - PRODUCTION QUALITY

• 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 cost.
• Build validation, schema enforcement and safe failure behaviour for outputs that may be well-formed yet wrong.
• Treat authentication, authorisation, secrets, input validation, OWASP awareness and audit trails as everyday engineering.

WHAT YOU'LL BE DOING - DELIVERY AND COST

• Deploy and operate on AWS using EKS/ECS patterns, RDS and S3 through Jenkins quality gates.
• Improve queries, caching, asynchronous throughput and right-sizing.
• Build queueing, rate limits and cost-aware orchestration for model-calling services.

WHAT WE'RE LOOKING FOR

• 5+ years in back-end engineering for production SaaS or platform systems.
• Real multi-tenant or enterprise-system experience covering isolation, permissions and auditability.
• Strong Python with FastAPI or comparable frameworks, solid SQL/PostgreSQL, and the ability to work at Java/Spring service boundaries.
• Production AWS experience, plus confidence with Git, CI/CD and Linux.
• Daily use of AI coding tools such as Cursor or Claude Code, and a considered view of where they help.
• A degree in engineering, computer science or a related field, or equivalent experience.

NICE TO HAVE

• Java/Spring depth alongside Python.
• Event-driven architecture, queues and workflow engines.
• Financial services or regulated-industry systems.
• LLM service integration patterns.

THE REALITY

• The work demands both pace and accuracy - autonomous systems are being built inside high-stakes live businesses where decisions carry real consequences, and waiting for full consensus isn't always an option.
• The systems will sometimes get things wrong. In this phase the London team creates the evaluations, visibility and safeguards that catch mistakes early, reduce them over time and stop the same failures recurring.
• The opportunity is to build the foundations of an enterprise-grade technology platform aimed at a multi-billion-dollar serviceable market, and to share in that growth.

INTERVIEW PREPARATION

Come ready to discuss three pieces of work:

• A service or API contract you evolved without breaking its consumers.
• A tenant or permission boundary you implemented and tested.
• A production quality, reliability or cost problem you diagnosed and improved.

LOCATION & WORKING PATTERN

• Based at Kings Cross, London, in office 3 days a week.
• Colleagues split between London and an India-based delivery organisation.
• Visa sponsorship is not available for this role.
• The interview process is intended to conclude within three weeks, and adjustments to interviews can be arranged on request.