When it comes to space travel, there’s a simple rule. If you didn’t bring it with you, you need to make it there. This usually refers to building material but it could apply to oxygen too.
In a study published in Plasma Sources Science and Technology, Portuguese and French researchers showed that both the temperature and pressure of the Martian atmosphere are good enough to efficiently produce oxygen. Astronauts would use nonthermal plasma to break down carbon dioxide, which makes up most of the Red Planet's atmosphere.
Not only would Mars' cold, CO2 rich atmosphere provide a stable, reliable supply of oxygen, but a source of fuel as well, as carbon monoxide has been proposed as to be used as a propellant mixture in rocket vehicles.