I am locked in a tiny vessel with limited oxygen supply, how do I maximize my survival time?

Control your respiratory rate (and your mind, panic increases respiration) while you figure out how to get help, and then keep practicing it while you wait for help or do what is necessary to free yourself. No unnecessary movement. Your O2 is your life and is being used up by you merely existing, so use as little of it as you can while still managing to do what you must to restore access to it.