This was a question posed in the Friday Tips thread, and my response got so long-winded that I decided it deserved its own thread!
Let me break down my mastering process real quickly for you.
Mix your track down so that you are really, really happy with it. As good as you can possibly get it. Use sidechaining, distortion, compression, and whatever else you need to get it sounding as loud as possible. Bounce it in as high resolution as you need, with the highest peaks below 0dB - most mastering houses will say below -6dB - and load it into a new project file. This may seem like an extra step, but it is important philosophically because then you are committed to having the track done, and when you open it up in a new project with just the audio waveform, you are committed to working on the mastering alone, and not going back to make compositional changes.
The first screen in Ozone is the EQ. I rarely use the EQ. It is a powerful and transparent EQ, but I find that any problems it can fix are better fixed in the mixing stage. For example, if there is too much bass, you should just turn down the bass in the mixing stage, not EQ it down in the mastering stage.
I rarely use Ozone reverb. It is a nice effect, and I can see how it would be useful for, for example, acoustic guitar compositions or vocal tracks, to give a sense of a room, but for our kinds of music, it's unnecessary. I worry about it muddying up my bass.
Ozone Loudness Maximizer
I typically keep the threshold at 0dB and the Margin at 0dB. If you want the track to be loud, you want the peaks to hit 0dB. That is the Margin. The threshold is where you want the maximizer to start limiting the sound. I usually keep that at 0dB as well, to preserve most of the dynamics of the track. You can experiment with different settings on it and see how it affects your sound. If you keep it at zero dB and run the master input in "hot" (above 0dB), you will notice that the effect is similar to lowering the Margin. I prefer to leave the margin at 0 and raise the input volume. I usually leave the character around the middle settings. You can play with that one too. Shorter settings will give a faster release time, and longer will be slower, which can sound a bit more natural.
Ozone Harmonic Exciter
This thing is the shit. I have no idea how it works, but it adds a lots of beautiful harmonic crispness and presence to the top end of your mix. I tend to use the default bandwidths (the 4 boxes at the top) and increase the Band 3 and Band 4 Amount controls by about 1.0, and then dial down the Mix controls until I get it where I want it. I don't touch the delay controls at all. I've heard that if you put a slight delay on everything except the highs, things sound a little more clear. You can try it, but I think it will make it harder to mix the track if you're beat matching. Basically I don't understand the delay effect enough to fuck with it effectively.
Ozone Multiband Dynamics
Here's my quick and dirty technique for boosting the perceived volume of your mix. Start with the bass band (default 20hz-140hz or so). Increase the band gain to 2.0dB. Don't touch the limiter settings. Turn the compression ratio up to 1.25-1.5 or so. Move the compressor threshold down to about -16dB. Visually, what you should see is the that the white dot at the top right of the volume diagram is going to realign itself with the top right corner of the box. When that is in line, your volume is not peaking out, so line it up visually. Then slowly increase the expansion ratio, up to 1.25 or so, and put the threshold of expansion at about -52dB. If you've done it right, you should have a series of 3 lines that connect the bottom left corner to the top right corner, and rise up a little bit above a diagonal. Essentially you've raised the perceived volume of that band by 2.0dB without allowing it to clip! Copy and paste (right click on the band for the contextual menu) these settings to the other 3 bands. Adjust them to taste. You may find that after compression, you need a little extra sparkle, so go back and tweak the settings on the harmonic exciter if you like.
Ozone Multiband Stereo Imaging
This one is great too. It takes the perceived stereo image of a band and widens it! It can also compress the stereo image (mix both channels to mono). I use this to expand the claps, synths, and whooshy noises that I want to sound super wide (high and mid-high bands). I typically leave the mid-low band untouched. I always put the low band all the way to MONO! Why? Because I want to make sure there are no phasing artifacts in the song! If you are mastering for a vinyl release, it is essential that the bass is mono! On a big soundsystem, you always want the bass to be mono. If you solo the low band and then move the slider to mono, you should not hear any change in the signal. If you hear a perceived dip in the volume, your left and right channels are out of phase in the bass range. This is a problem that must be corrected in the mixdown. Go back and solo each instrument and make sure there is no bass that has a phase delay or any other "spreading" plugin on it. Spreading should only be used on frequencies above ~500hz. You may need to use a low cut EQ on sounds like snares or synths to make sure they are not introducing any phasing artifacts in the low end. The stereo imaging is great for identifying this kind of problem, so this is a situation when it is ok to use ozone during the mixdown process. But use it and then fix the problem, bounce the song, and reopen Ozone again for the final master!
A/B-ing just means comparing one thing to another. Now would be a good time to start A/Bing your master with the unmastered mix. Set up a duplicate channel in your DAW with the mixed audio track on it, but without the master. Listen to the mastered track (solo'd), and turn the volume down so the peak is at -6dB. Turn up the unmastered track (solo'd) until the perceived volume of the track matches, as close as you can, the perceived volume of the mastered track. This may be (indeed, should be) peaking above the level of your mastered track. This A/B test will help you as a reference point when deciding on certain choices you must make as a mastering engineer. For example, when doing multiband compression, you have to choose between preserving the dynamics of a song, and making the song sound loud. To a degree, other producers and engineers have already made this choice for you. If you play a quiet song in a set with a bunch of loud songs, it will be the song where everyone goes to get a drink. Still, you should try to preserve the dynamics as much as possible. A/B-ing will give you a frame of reference for how much perceived volume you are adding to your track, as well as for effects that you can go overboard with if you're not careful, such as the harmonic exciter.
I'm still learning a lot of stuff about mastering, but there is my basic, functional guide to what I do on my tracks. Mastering is not magic, and the mixdown is the most important part of the process.