Emergency oxygen comes in differing tank sizes, and internationally, you may encounter different valve configurations, so it’s a good idea to check the local standards when traveling. Ideally, carry a big enough supply to keep a patient on pure 0xygen until in the hands of emergency medical care. However, some very remote dive destinations may make this impractical or impossible; carry as much oxygen as you reasonably can. Some oxygen is better than none at all. For general purposes, 637 liters of oxygen (22.5 cubic feet; even imperial system countries usually measure medical oxygen in liters), can be expected to last approximately 40 to 50 minutes, depending upon whether used with a non resuscitator demand valve or continuous flow.

Like your first aid kit, your oxygen equipment needs a case that can withstand the rigors of diving, ideally one in which you can store your equipment set up and ready to go. Most commercially available oxygen systems for Scuba divers come equipped with a suitable case. Most airlines won't let you bring a pressurized oxygen tank aboard the plane when you travel. If you frequent distant destinations that may not have oxygen on site (i.e., remote locations that lack  Bali scuba dive resorts) you can also get systems that have everything except the oxygen tank. Instead, you rent the oxygen tank at your destination and bring it to the dive site.