The 3DO M1 Opera platform ran an OS called Portfolio. For the time a quite advanced operating system which supported preemptive multitasking, dynamically code loading, supervisor/user process separation, memory fences (MMU), threading, message passing, a filesystem, high level IO abstractions, etc. Portfolio was created to help with developing on the platform naturally but also acted as the foundation for providing an abstraction to the hardware so that manufacturers had more flexibility in their design and provide for backwards compatibility with future systems (that never came.)
On 2022-01-07 a snapshot of the M1 Opera's Portfolio OS from 1995-02-10 was uploaded to Archive.org by user EagleSoft.
This repo is a copy of the Portfolio OS source code which was released by EagleSoft on 2022-01-07. While I wasn't responsible for the release of the Portfolio source code I did speak with the individual who did so prior to release, maintain https://3dodev.com, and maintainer of the libretro 3DO emulator Opera. This repo is in the least a GitHub hosted backup but may be a starting place for future development.