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BestOS

Software for the next stack.

Web, mobile, and desktop products — built with modern systems thinking, shipped with discipline. Remote across Europe, the UK, and the USA.

Start a projectSelected work
ReactNext.jsTypeScript.NET CoreVueReact NativeAngularRedisDockerPostgreSQLReactNext.jsTypeScript.NET CoreVueReact NativeAngularRedisDockerPostgreSQL
4+years shipping product
6countries served
Web→Desktopfull product surface
NDA-readyhealthcare & pharma

Services

Capabilities

Modern product engineering with fixed scope after discovery.

  • 01

    Custom web applications

    React and Next.js apps, dashboards, and admin panels built for real users.

    from $800
  • 02

    Websites & landing pages

    Fast business sites with clear offers, strong SEO basics, and clean design.

    from $400
  • 03

    Booking & appointment systems

    Constructors for services, staff calendars, time slots, and client booking flows.

    from $900
  • 04

    Mobile app features

    React Native features, fixes, and releases for iOS and Android.

    from $600
  • 05

    .NET APIs & integrations

    Backend services, third-party integrations, and stable production APIs.

    from $700
  • 06

    MVP in 2–4 weeks

    Scoped MVP with milestones, demos, and a path to ongoing product work.

    from $1,500

Work

Selected systems

FitTracker product screens plus booking, healthcare, and cross-platform delivery. Client UIs stay confidential where required.

FitTrackerProduct · Mobile

FitTracker

Own strength analytics product — gym-floor logging, training history, muscle recovery map, and analytics that drive the next block.

Founder product · liveOpen live product
Appointment booking constructorSaaS · Booking

Appointment booking constructor

Builder for services, staff, schedules, and client bookings — calendar slots, forms, and live preview for clinics and service businesses.

Product UI concept
Healthcare web platformHealthcare · Frontend

Healthcare web platform

Senior frontend on clinical workflows, complex forms, and reusable React UI for day-to-day medical staff.

Illustrative mockup
Pharma & eHealth systemsPharma · Full-stack

Pharma & eHealth systems

Full-stack .NET Core APIs, Vue/React modules, queues, cache, and regulated-domain integrations.

Architecture · NDA-safe
Cross-platform productsWeb · Mobile · Desktop

Cross-platform products

Web, mobile, and desktop feature work across React, Vue, Angular, React Native, and .NET.

Multi-surface delivery

Process

From brief to launch

Visible progress every week.

  1. 01

    Discovery

    A short call to clarify goals, constraints, and the smallest useful first release.

  2. 02

    Scope & offer

    Fixed milestones, timeline, and price before development starts.

  3. 03

    Build & demo

    Regular demos, clean code, and production-minded architecture.

  4. 04

    Launch & support

    Release help plus optional monthly maintenance when you want ongoing velocity.

Studio

BestOS · Valentyn Riabinchak

Senior full-stack engineer building with React, Next.js, Vue, React Native, and .NET. Architecture first, clean interfaces, long-term maintainability.

Ukraine · Poland · Czechia · Slovakia · United Kingdom · United States

View Upwork profile
FitTracker app screens — logging, history, body map, analytics

Contact

Start the next build

Upwork for contracts, or call for a short discovery chat.

Message on UpworkCall 099 288 0228

Remote · Mon–Fri 09:00–20:00 · Sat 10:00–16:00 (Europe/Kyiv)