How "serious" an OS is this? Is it a theoretical construct, or does it need to fully utilize a modern system?
If you could get away with writing in 16-bit real mode, your work would be miles easier -- you could write a "hello world" without having to worry about disk drivers, keyboard interrupts, timing controllers, or any protected-mode histrionics by relying on BIOS I/O calls. Once you've got your "hello world" working, you can start building the structure from there. I'd almost say you have to start this way, getting a tinkertoy version of your OS running in 640K might happen before you manage to properly switch into real mode. If people really like it, they could reinvent it in protected mode themselves, like how Minux inspired Linux.
Either way though, getting a bootloader to load a kernel is way different than getting an OS to load a program. Some "magic" code is needed, and you need to put the right things in the right places. Play around with the GRUB bootloader, and have a look at the source code for
memtest86 -- it's a small program that loads and runs raw from the bootloader too.