And the oceans are incredibly vast, so they provide most of the world’s oxygen! Obviously it’s hard to get a precise number but 50-70% is the accepted range.
There are many reasons to plant trees in the city but local oxygen supply isn’t one of them. Mostly trees look nice, and make people feel better by their presence. They also have a significant cooling effect, something a steamy tank full of warm algae definitely won’t help with on a summer day.