Let's say there's a viable city of 150,000 people on Mars, the colony has been around for 50 years or more. Private corporations have funded most of it and they have a large presence on planet. I know you can use frozen water, but let's say we don't want to "waste" our water deposits to make oxygen.
There's the MOXIE project, converting CO2, and this is a real project. I saw in Andy Weir's book Artemis that O2 was produced as a byproduct from manufacturing aluminum (I am not sure how, have not read the book.)
I'm guessing there are other options, but not having a lot of luck in my search. Let's assume terraforming hasn't happened yet, either. How can we produce enough O2 to sustain a large colony?