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You need to pump air into a plane to provide oxygen. That implies that you need to let air out. The pressurisation system in planes is able to deal with a moderate amount of air exiting the aircraft without losing pressure, and the lower the altitude the easier that is.

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You can have a hole the size of a quarter with no effect on cabin pressure.

This is why the fear that a bullet fired in the airplane will pop it like a balloon is unfounded. Poking a hole in the fuselage won't do anything. A bullet would have to hit and disable something critical (and there's always a backup for critical parts).

Not oxygen, air. The outflow valves on modern aeroplanes are almost the size of the windows themselves. They would progressively close to maintain cabin pressure.

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