Dang, wish I had seen this post sooner. I just downloaded a free DRM-stripping program, but it is free only for a limited time. I don't know if it will still be available by the time you see this, but here's the link.
Edited to add:
I guess it doesn't matter, since I just checked the company's website and it doesn't list .oma files as convertible. But wikipedia says Sonicstage 3.4 includes an option to remove DRM from your files, so yay?
Thanks all. Been trying to move the music library from the old to new computer. Seems like for these last few, it may simply be a matter of digging the CDs out from the bottom of the closet.