Umm, okay.
Just a checklist, to make sure you can cover everything that an operating system requires:
A simple (!) operating system is required to:
* Run one or more processes simulaneously (MS-DOS v3 and below ran only one, but newer OSes multi-task)
* Provide a fair and efficient multi-tasking environment if multi-tasking is supported (Interactive tasks should be given higher priorities, etc)
* Manage memory for each task - a process should not have access to memory it doesn't own, but multiple processes should be able to share common code segments in memory.
* Manage I/O for each task - the OS provides the interface between the software and the hardware (Roughly speaking)
* File management - providing the correct file to a process when requested, and managing the file list as a whole to prevent fragmentation and the like.
* System security - a user should be able to access his/her files and resources but not those that are not shared and belong to other users. A system for auditing is also useful.
If you still feel up to it, the book 'Operating Systems (3rd Edition)', by W Stallings (Prentice Hall, 1995) is an exellent reference.
If you manage to work out a clean, efficient operating system that works, please let me know and I'll likely be among your customers.
Good luck,
White Fire