There are 4 repositories under os-dev topic.
A list of operating systems and stuff
Source for the little book about OS development
The Operating System (for x86_64) written in Crystal as much as possible
The collection of useful resources for OS development
A 64bit hobby operating system written in C++
A Unix/Linux like operating system
RADII is a UEFI bootloader for the x86_64 architecture. It is made with the goal of understanding how modern desktop computers work, from hardware to radio buttons :^)
Tutorial: getting started with OS development in assembly and C/C++ (GCC) on Windows
Open Source Developer
A hobby kernel developed from scratch using i8086 assembly
A Journey in Creating an Operating System Kernel: A book that teaches the reader how to create an operating system's kernel called 539kernel. You can read the book freely in https://539kernel.com/
A project done for OS course, extending a kernel from https://github.com/dthain/basekernel, to implement threading capabilities, give an API for threading, and implement various scheduling techniques.
Custom operating system made in C
A Simple OS for literally everything.
This is a simple Operating system made with limine bootloader, it is planned to be a command line (CLI) operating system. uses BIOS