If you're trying to learn the "ins and outs and subtleties" and what not of mastering, I can't think of a worse place to start than with Ozone. Honestly.
A simple EQ and a simple broadband compressor. If you can't do 90% of what you need with those two tools, it's not the fault of the tools... And throwing in a bunch of stuff that mastering engineers never use -- Maul-the-band compression, multi-band "stereo enhancement" (a crazy Haas filter), "mastering reverb" (that still makes me crack up) and the like...?
Everything you need is in Reaper. Once you get to a point where Reaper isn't doing what you need, then start looking for other tools.