To the first approximation, you don't have to --- OS/2 2.x's DOS support is excellent, and your DOS program will probably just work; similarly, OS/2 2.x supports Windows 3.0 (and soon 3.1) programs. See the OS/2 User's FAQ for details.
[That was Barry Jaspan's opinion. I believe that you should make every effort to recompile your existing DOS programs for OS/2 2.x. They will run faster in many cases, and both (a) use less memory and (b) be able to use more memory than their DOS counterparts. - Jeff]
If you truly want to port your DOS program over to OS/2, then study the libraries available to you. The core code (if you wrote it correctly) will probably not change much. You will just have to change the user interface stuff.
If your program is a real simple one that uses standard input and output, then you will probably not make very many changes to your program when converting it to OS/2.
You should also realize that neato and nifty DOS tricks (like grabbing an
interrupt whenever you feel like it, or writing directly to almost anywhere)
are completely out of the question.
Related Information:
How can I port my Windows program
to OS/2?