“Before you start dreaming of a subterranean race of Martian monsters, it’s important to know that this isn’t a smoking gun for life on Mars. In fact, it’s far from it. There are natural processes that can generate oxygen in the absence of life, and since we have yet to find evidence of life on the Red Planet, yet can’t rule it out, scientists are considering all possible options.”
This still doesn’t answer what is “using” it once it is made.
My highly unscientific guess is that nothing is using it. Given the lack of atmospheric pressure, I'm gonna say that it all just dissipates out into space.