O2, Ammo, Meds & Exoskeleton Armour are treated as ‘one time use consumable’ items. This means when you remove an item from your personal load-out the item is discarded.
It does not matter if the item was full, partially used or empty. Once removed it is discarded as ‘used’. This means a certain amount of forethought and planning must take place prior to exploring a derelict.
You can only use what you take with you. So it would be pointless starting an exploration with say a half full O2 tank or perhaps with only one or two meds left in your med kit.
This method is employed to say you the trouble of micro managing partial and near empty kits. But it does place the onus on you to plan ahead.
Space Suits, Side Arms and Tools are not classified as consumable.