Yes, in the sense that the game software would interface with hardware (more) directly, service interrupts, and manage its own low-level i/o, you might could sa

Yes, in the sense that the game software would interface with hardware (more) directly, service interrupts, and manage its own low-level i/o, you might could say that it was its own OS. Some software, like GEOS, would completely replace KERNAL with its own OS, API, etc. (KERNAL was very simple compared to what we think of as operating systems today -- only 8K, no virtual memory, and no processes.)