bike rack and a single pannier. It will easily hold a small oxygen tank if your hose is long enough. Or just use the rack and bungee the cylinder to the rack. You could also try one like this $15 rack bag if the tank is small enough. Bicycle Bike Seat Shoulder Bag Rear Tail Rack Pannier Cycling Handbag Pack | eBay

I don't think you would find carrying a metal cylinder in a back pack to be very comfortable.