Sure, there is a gradient of complexity here, first you can just do your stuff in kernel in a single thread, then you'll probably want to load programs and exec

Sure, there is a gradient of complexity here, first you can just do your stuff in kernel in a single thread, then you'll probably want to load programs and execute them in userspace [0], then you'll want multitasking and task scheduling, memory isolation and process syncronization etc. You can call any of those steps an OS of course, its just I personally find earlier steps not that interesting. Another one great resource (that actually inspired me to tinker with all this awesome stuff) is this book [1] [0] https://wiki.osdev.org/Getting_to_Ring_3 https://wiki.osdev.org/Getting_to_Ring_3 [1] https://pages.cs.wisc.edu/~remzi/OSTEP/ https://pages.cs.wisc.edu/~remzi/OSTEP/