I have used O3 generators in the past mostly to knock down that (musty) basement smell. They work and WORK WELL. You really have to be careful when using them as they're very toxic in higher doses as Kid Montana posted. It's obvious that you wouldn't want to be in the same room when it's running. Most if not all generators will have different programmable cycles that you can use. I would have mine run for 10mins every 60min for 72hrs in a 400sq ft area. That seemed to control the musty smell during the rainy season. You can run it for 60min, but be sure to leave the house for a few hrs while it's on.
Another item to keep in mind is the mess. O3 knocks particles out of the air causing this extremely fine black powder that is a pain in the ass to clean up. You'll really see the black powder on the plates when you clean the machine after a week of use. You could even say the black powder substance is magnetized as it likes to stick to itself and other objects (your floor).