Why do you want to use pure oxygen? They ought to be fine without if bagged correctly. They aren't super large are they? Pure oxygen will cut down on size and weight mind you. This is why I used it to send a bulk lot of cichlids from Blenheim to Hamilton via hand luggage on 2 flights 8)
Without using pure oxygen;
Double bag them (that is, one bag inside another) and use 1 1/4 water to 3/4 air. Do not put too many in each bag. If they are larger than 6cm, have 1 per bag only. Do not feed them for 2 or 3 days before they go. This cuts down on waste produced en route
Pack scrunched up newspaper, towelling, or something around the bags when you put them in a box to make sure the bags can't move around or fall over.
PS. I got my pure oxygen from the local emergency department (I know a Dr there and he loaned me a small bottle of the stuff, complete with a bit of airline).