A new product or application
I can help shape the first version, design the architecture, and build a foundation that can grow with the product.
Available for freelance projects and team collaboration
Backend systems, mobile apps and web solutions
I am a full-stack developer with a strong focus on backend and mobile development. I specialize in Kotlin, Spring Boot and native Android applications built with Jetpack Compose.
I design REST APIs, backend systems, mobile applications, web products and fast business websites. I also work with Go, React, Next.js, TypeScript, Astro and Docker.
How I can help
Support at different stages of a project
I can help with a first product version as well as the continued development of an existing solution.
A module, backend or integration
I can deliver a module, integration, application, or backend without requiring the team to guide every technical detail.
Developing an existing project
I can enter an existing codebase, clean up parts of the app, add features, and reduce the risk of future changes.
For recruiters
Hiring a developer for your team?
See my technical profile, key areas of experience and public projects.
- Backend, Android, web applications and business websites
- Kotlin as the main technology, with Go as an additional backend technology
- Public projects and a technical profile without NDA-protected work
Services
Services
I build solutions matched to the project scope — from backend systems and mobile apps to web systems and fast business websites.
Backend, APIs and integrations
I design backend systems primarily with Kotlin, Spring Boot and Ktor. I build REST APIs, authorization, data models, external service integrations and synchronization mechanisms. I also use Go for selected backend services.
- Kotlin
- Spring Boot
- Ktor
- REST API
- PostgreSQL
- Go
- Docker
Native Android applications
I build native Android applications with Kotlin and Jetpack Compose. I connect them with backend systems, local databases and synchronization while supporting offline use and good performance.
- Kotlin
- Android
- Jetpack Compose
- Material 3
- Coroutines
- Room
Web applications and internal systems
I build web applications, administration panels and customer-facing or internal systems. I can deliver the frontend, API integration, backend, database and deployment.
- TypeScript
- React
- Next.js
- Tailwind CSS
- REST API
Business websites
I build fast business websites, landing pages and portfolios with responsive design, technical SEO, good performance and useful integrations.
- Astro
- Next.js
- WordPress
- TypeScript
- Tailwind CSS
Skills
A short technology list
These are the technologies I use most often, selected according to the needs of each project.
Explore full stackBackend
Backend systems, APIs and data models designed for clarity and continued development.
- Kotlin
- Spring Boot
- Ktor
- Go
- PostgreSQL
Mobile
Native Android apps with smooth interfaces and platform-aware implementation.
- Android
- Jetpack Compose
- Material 3
- Room
Web
Websites and applications that load quickly and work well on mobile devices.
- TypeScript
- React
- Next.js
- Astro
- Tailwind CSS
Infrastructure
Environments and deployment automation that support predictable project development.
- Docker
- Linux
- GitHub Actions
- Redis
Projects
Selected Work
Selected work and self-directed projects showing how I think about architecture, security, interface design, and deployment.
LibrePass
An open-source password manager designed like a full product: end-to-end encryption, zero-knowledge architecture, client applications, and a Kotlin/Spring Boot backend for multi-device synchronization.
- Kotlin
- Spring Boot
- Jetpack Compose
- Docker
- Integration Testing
Oskar Karpiński
This portfolio website - a fast, statically generated, bilingual personal site built with Astro 6, Tailwind CSS v4, and TypeScript.
- Astro
- Tailwind CSS
- TypeScript
- i18n
Have a project to build or improve?
Write if you need a backend system, Android app, web system, website or integration with an external API.