The C-5 is equipped with emergency masks at all primary crew stations (like airliner cockpits) and the relief and courier stations aft the main cockpit, portable tanks/masks to allow crew to walk around the cargo bay and drop down masks (airliner like) in the aft troop deck and other places. The normal and emergency oxygen system is described here in section 13-1.
http://www.usaf-sig.org/index.php/re...7:c-5-handbook