Oxygen is used up just by breathing. Having an O3 machine running will not increase or decrease the supply of oxygen so fresh air is required no matter. Only if doing an ozone shock treatment on the structure should it be sealed up but these are way high concentrations of O3 not suitable for people, plants or animals.
I would think the perfect environment would receive a 25% flesh air per hour and an ozone machine large enough or a space small enough to maintain 200 mg/hr per 1000 cubic feet.
As an example if you had a 10 x 10 x 10 room with 25% per hour fresh air you would need an additional 50 mg/hr ozone output or 250 mg/hr.
My environment is not perfect but based on what is practicle. In my case it is to leave the bathroom exhaust fan running 24/7 which I only guess changes out more than 25% of my air every hour. In my recent experiment with O3 I was using a 350 mg/hr machine in a 500 cubic foot area along with a +25% fresh air per hour effectively giving me a dose of 450mg/hr per 1000 cubic feet of air which is more than twice that of what I just contradicted in action of what I said was ideal, ideal being a healthy 24/7 environment of 200 mg/hr per 1000 cu/ft.
But,...... but I was mega-dosing!