An ozonator usually doesn't make much sense for an outdoor residential pool or for a low-use (once a week or less) residential spa. The reason is that ozone destroys chlorine so if there isn't a lot of bather waste to get rid of then it just ends up using more chlorine than it saves. An ozonator is useful in more heavily used residential spas (used every day or two) and in most commercial/public pools and spas where the bather load is much higher than typical residential pools. It also is useful for indoor pools though usually UV is used in that situation. It is also useful (as is UV) in commercial/public pools and spas to handle killing Cryptosporidium parvum which is chlorine-resistant but that's not something you'll find in your residential pool or spa unless you invite someone over with diarrhea that is carrying the pathogen.

For outdoor residential pools, chlorine handles bather load just fine and the UV in sunlight even produces the same hydroxyl radicals and hydrogen peroxide that ozone can produce.