But also, any growing plants - you could compute for every kg of dry plant matter, how much oxygen is released, as the dry plant matter is almost entirely carbon produced from breaking up CO2.
So unless you're collecting tens or hundreds of kilograms of dead plants from your roof every year, it isn't producing much oxygen.
Because op said "Plants don't produce any useful amount of oxygen"
And I am aware, that room plants do not really add meaningful O2 to the room, their value is in humidifying.
> Algae are in the plant kingdom, but technically they are not plants. ... Algae range in size from microscopic to meters long and from single-celled to complex organisms that rival large plants. These organisms may look like true plants, but unlike plants, algae do not have roots or true stems and leaves.
Either way, I wasn't thinking of algae or aware that they might be net oxygen producers for the atmosphere.