I think what GP was getting at is a big part of an OS is resource management (process permissions and isolation) and providing abstract interfaces over hardware

I think what GP was getting at is a big part of an OS is resource management (process permissions and isolation) and providing abstract interfaces over hardware (filesystems vs raw block storage). So beyond the low level drivers that directly interact with hardware, an OS differs in how you ... drive the drivers. What your HAL looks like, and then what the abstractions above that look like. It's by that definition that e.g. Android is considered a different OS from GNU/Linux. Same kernel, but the hardware is exposed much differently to end-user applications.