100% can vouch for the efficacy of ozone machines!
We bought an 1800 sf house from a family of chain smokers who also owned elderly incontinent cats. I could barely stand to be in it when we first viewed it because of the stench. After tearing out the carpet, we purchased three ozone machines and ran them for several multi-hour stints over the course of a couple weeks (while we were out of the house), and even before any scrubbing or painting they killed a huge chunk of the odor.
We eventually washed all the walls down with TSP, primed with Zinsser shellac, and painted, and the smell is totally gone.
Bonus: great for killing bugs if you’ve got a house in the woods. 