You can easily make an OS with Freepascal. You can do that with Oberon too but with Oberon you are a bit more limited as you, AFAIK, need to have a compiler in the bootloader which limits you to the Oberon Bootloader.

With Freepascal you have pretty much unlimited possibilities. I've made my os work on x86 and x86_64.
If you want anymore information about OS development you can go to osdev.com or eventually hit me with a PM