Change the functionality of the portable oxygen generator to be a device that the player places on the ground before activating it. When turning it on it then becomes a stationary spot that the player can come back to in order to fill up with oxygen. Similar to how in wrecks we periodically come across air tanks releasing bubbles we can stand by to replenish our oxygen supply. This would be useful for exploring a new area having a central spot to come back to as long as it remains powered.