Raspberry Pi OS is described as '(previously called Raspbian) is the Foundation’s official supported operating system. You can install it with NOOBS or download the image below and follow our installation guide' and is a very popular Linux Distro in the os & utilities category. There are more than 100 alternatives to Raspberry Pi OS for a variety of platforms, including Linux, Self-Hosted, BSD, Windows and Mac apps. The best Raspberry Pi OS alternative is Ubuntu, which is both free and Open Source. Other great apps like Raspberry Pi OS are Arch Linux, CachyOS, Zorin OS and openSUSE.
Ubuntu is a community developed, Linux-based operating system that is perfect for laptops, desktops and servers. It contains all the applications you need - a web browser, presentation, document and spreadsheet software, instant messaging and much more.
Its not an OS for Raspberry Pi
Arch Linux is an independently developed, x86_64 optimized GNU/Linux distribution targeted at competent Linux users. It uses pacman, its home-grown package manager, to provide updates to the latest software applications with full dependency tracking.
Arch Linux has a port available that will run on the Raspberry Pi.
CachyOS is designed to deliver lightning-fast speeds and stability, ensuring a smooth and enjoyable computing experience every time you use it. Whether you're a seasoned Linux user or just starting out, CachyOS is the ideal choice for those looking for a powerful...
Linux-based OS designed for familiar user experience, mimicking Windows or macOS, supports dual-boot, runs Windows apps via WINE, pre-installed essential software, customizable desktop layouts, broad hardware compatibility, and privacy-focused updates.
openSUSE is a free and Linux-based operating system for your PC, Laptop or Server.
Manjaro is a user-friendly Linux distribution based on the independently developed Arch operating system. Within the Linux community, Arch itself is renowned for being an exceptionally fast, powerful, and lightweight distribution that provides access to the very latest cutting...
Kubuntu is an official "flavor" of Ubuntu that uses the KDE Plasma desktop environment. This gives it a completely different look and feel.
elementary OS is a free, fast and aesthetically pleasing operating system based on Ubuntu .
Versatile OS with universal app architecture, free first-year offer for Windows 7 and 8.1 users, redesigned mouse-touch UI, virtual desktop, Microsoft Edge, upgraded security, and enhanced graphics for gaming, supporting fingerprint and face recognition login.
After October 14, 2025, Windows 10 will no longer receive free software updates, technical support, or security fixes. Microsoft suggests to upgrade to 
Windows 11 : https://blogs.windows.com/windowsexperience/2024/10/31/how-to-prepare-for-windows-10-end-of-support-by-moving-to-windows-11-today/
It's not open source, not free and because it's not a Linux distribution, it works in a different way.
Free Linux-based OS from KDE community, emphasizing safety, functionality, and advanced desktop technologies, integrating Plasma and KDE apps for home, enterprise, public, or pre-installed computer use, prioritizing updates, usability, and customization.
FreeBSD is an operating system used to power modern servers, desktops, and embedded platforms. A large community has continually developed it for more than thirty years. Its advanced networking, security, and storage features have made FreeBSD the platform of choice for many of...
FreeBSD has a port available that will run on the Raspberry Pi.
EndeavourOS is an Arch-based distro that provides an Arch experience without the hassle of installing it manually for both x86_64 and ARM systems. After installation, you’re provided with a lightweight and almost barebones environment ready to be explored with your terminal...
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Ubuntu CAN be indeed an alternative. See: https://ubuntu.com/download/raspberry-pi .