In this paper we report on the open-source software platform for satellites called Cube-OS, designed by the ARC Training Centre for CubeSats, UAVs, and their Applications (CUAVA). Currently satellite developers have a limited choice for the flight software development. The closed source or proprietary environments (e.g. GOMSpace) and available open-source systems (e.g. FreeRTOS) are very time-consuming, while commercial software licence (e.g. ISISpace) charge large sums of money. Leveraging the rapid development of single board computers, we propose the use of a Linux base open-source platform called Cube-OS. The platform consists of the simplified Linux operating system Cube-OS as well as Core Services, Bus Services, HAL and examples of APIs, Services and Mission Applications to interface with payloads. Cube-OS’s Linux nature and user-friendly examples improve software integration times and enable complex CubeSat missions with many payloads (e.g. rideshare projects like WS-1). Cube-OS builds on our experience from the CUAVA-1 satellite (deployed in Oct. 2021) and will be running on both CUAVA-2 and Waratah-Seed 1, with improved reliability, performance, and simplicity.