Depends how dense the oxygen is ;) It wouldn't, but there are methods to retain it. If we had the capability, which we definitely don't, currently. On earth we

Depends how dense the oxygen is ;) It wouldn't, but there are methods to retain it. If we had the capability, which we definitely don't, currently. On earth we have a magnetosphere that protects our atmosphere from erosion from the deluge the sun gives us (hello Auroras!) but Mars doesn't have that. It's size is an issue too plus it's distance and orbit around the sun now. It didn't used to be so barren, it had massive lakes and oceans and possibly life once upon a time. To terraform it now would take a level of engineering that we don't have, but could, if we manage to not nullify ourselves in the next 200 years.