Senior Software Engineer - Core Systems

Onmo; Greater London; On-site; GBP 100,000 - 180,000; Full time

Job summary

Onmo is looking for a senior backend or full-stack engineer to help build and operate our core banking capabilities in London. You will design, implement and run money movement, interest calculation and statement generation within distributed, event-driven systems.

You’ll own foundational services, contribute to architecture decisions with the tech lead, and mentor engineers across the function while shipping reliable, well-documented code in a fast-paced fintech environment.

Qualifications

• Proven experience designing and running distributed systems in production.
• Strong Node.js/TypeScript and AWS expertise across serverless and containerised workloads.
• Infrastructure as code with Terraform; CI/CD as a habit.
• Track record of raising engineering standards through reviews, patterns, tooling or mentoring.
• Write clean, tested, maintainable code and improve system understandability.

Responsibilities

• Design, build and run core banking capabilities that are accurate, monitored and dependable.
• Build new core systems and migrate off vendors safely with parallel runs and clean cutovers.
• Own and evolve shared services, modernising patterns and improving reliability.
• Set patterns through RFCs and pave paths with libraries and guides.
• Review colleagues' contributions to core-owned services to keep high standards.
• Write tested, correct, documented code and mentor engineers.
• Shape technical direction with the tech lead on architecture and build-versus-buy decisions.
• Make systems self-serve so other teams can ship on them independently.

Skills

Correctness and reliability

Distributed, event-driven design

Documentation and knowledge sharing

Clear stakeholder communication

Startup pragmatism

Ownership of foundational systems

Ability to operate under ambiguity

Tools

Node.js/TypeScript

AWS (serverless & containers)

Terraform

CI/CD practices

Job description

ABOUT THE ROLE

We're building the core banking capabilities that power everything at Onmo - the systems that hold accounts, move money, calculate interest and generate statements. The Core Systems team owns these capabilities end to end, along with the shared services the rest of engineering builds on.

You’ll help build new core systems from the ground up as we grow and pursue a UK banking licence, evolve the services every team depends on, and set patterns that get adopted across engineering. Your work reaches customers through every journey at once.

TECH STACK

• Backend: Node.js / TypeScript on AWS - serverless (Lambda, API Gateway, DynamoDB, S3, SNS, SQS, EventBridge, Kinesis) and containerised compute (ECS, ECR, EC2), with more of the latter as our core systems grow

• Frontend: React / Next.js

• Infrastructure: Terraform, GitHub Actions CI/CD, Docker

• Observability: Datadog, Grafana, Prometheus, Loki

RESPONSIBILITIES

• Design, build and run core banking capabilities - money movement, interest, statements, reconciliation - that are accurate, monitored and dependable

• Build new core systems and migrate off vendors safely: parallel runs, reconciliation, clean cutovers

• Own and evolve our shared services: modernise patterns, improve reliability and performance, keep them easy to build on

• Set patterns that get adopted across engineering through our RFC process - landed as libraries, guides and paved paths, not one-off fixes

• Review other teams’ contributions to core‑owned services: quick, helpful reviews that keep the bar high without blocking anyone

• Write code that sets the standard - tested, correct, documented - and mentor engineers across the function

• Help shape technical direction with the tech lead: architecture, build-vs-buy decisions, engineering standards

• Make our systems self-serve, so other teams can ship on them without needing us

ABOUT YOU

SKILLS / ATTRIBUTES

• You care about correctness - edge cases, idempotency, retries, reconciliation - because this code moves money

• You're comfortable designing distributed, event-driven systems and reasoning about failure modes and data consistency

• You'd rather build the paved path than fix the same problem five times, and you write things down so others can follow

• You communicate clearly with engineers, product and business stakeholders, and review others' work firmly and kindly

• You're pragmatic in a startup: happy across greenfield builds and a legacy estate mid-migration, choosing boring technology when boring is right

• You want real ownership of foundational systems

• Can operate effectively under ambiguity

QUALIFICATIONS / EXPERIENCE

• Proven experience as a senior backend or full-stack engineer building and running distributed systems in production

• Strong Node.js/TypeScript and AWS, across serverless and containerised workloads

• Infrastructure as code with Terraform; CI/CD as a habit

• A track record of raising engineering standards around you - through reviews, patterns, tooling or mentoring

• You write clean, tested, maintainable code and leave systems more understandable than you found them

BONUS SKILLS

• Fintech, banking, payments or ledger experience - reconciliation, card rails, double-entry accounting

• Experience with core system migrations or vendor replacements

• Experience getting real leverage from AI-assisted development tooling

• Start-up experience

FCA Compliance & Consumer Duty

At Onmo we all take collective responsibility for our individual roles in creating the best outcomes for our customers. In this role that involves:

• Following the FCA Conduct Rules:

  • You must act with integrity

  • You must act with due skill, care and diligence

  • You must be open and cooperative with the FCA, PRA and other regulators

  • You must pay due regard to the interests of customers and treat them fairly

  • You must observe proper standards of market conduct