I think you should try and forget you ever heard the term "mastering" at this point. It's a part of the audio engineering process has become extremely misunderstood, and a phrase that is often misused (usually used to reference techniques and processes that are ACTUALLY mixing).
Don't even THINK about mastering yet. Put Ozone away and don't take it out until you're 100% happy with the sound of your song(s), until you're absolutely sure that you can't make them any better by using the best playing/singing technique, using the best recording practices, and mixing to the best of your ability.
At this point I usually put my song(s) away for a while. Don't listen to them, don't tweak them or play with them at all. Once I'm happy with the mix I try to forget about what the song sounds like (basically try to erase any opinion you have of the song, which is probably impossible, but it's worth a try).
Then I sit down with the rendered .wav file and just listen to it in my car, in my house, on my iPod, in my garage when I'm working on my car and take notes about what can and can't be changed, keeping in mind how different it sounds on all of those systems. Also, if I have more than one song, I try and think about what order they should go in, how loud they are when compared to each other. That's for my own stuff.
If I'm using someone else's material it's a lot easier to be subjective about how it sounds. I care less about how much time went into creating it so cutting things out, changing how something sounds or outright recommending that something be re-recorded, or taken out of the song isn't an issue.
By and large I don't really consider any of this to be mastering. It's really just an extension of the mixing process. If I want to master something I will bring it to someone who I know is more experienced than I am and who has better equipment and space than I do.