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ChrysaLisp
Parallel OS, with GUI, Terminal, OO Assembler, Class libraries, C-Script compiler, Lisp interpreter and more...
I agree except the older Pi's peripherals are memory mapped (though largely undocumented) and/or use a "mailbox" system, so you don't need to go through USB. The Circle project might be a place for OP to look into.
For the Raspberry Pi I'd start with this "hello world" project . It gets you booted up, and control transferred to your OS in C starting at int main(). It provides printf functionality to the screen (and basic 2D graphics), access to many of the hardware registers, hardware timer and interrupts. Plus some global variables and functions relating to the system (made available in the .S startup loader).
I'll give another OS project to contribute to where the developers will help you a lot with your understanding of OS development: https://github.com/vygr/ChrysaLisp