I have a lot of bags but none designed for an oxygen tank. However I live in a retirement community and I see a lot of people with them. I would think that any good backpack would allow you to attack the oxygen tank using its webbing or an open compartment. Many have compartments for iPads these days. Heck, I have a winter jacket with an interior iPad pocket and concealed holster in the pocket.
Any compartment can become a concealed holster if it is large enough for your gun. Blackhawk sells an inexpensive universal holster with a mag holder that attaches to a strip of Velcro loops. The industrial strength ones they sell in Home Depot and Lowes work very well. I have used them to hold heavy radio equipment. Just put a strip inside any compartment and attach the holster and you are good to go. However, a gun in a backpack is not very accessible. There are some fine belt, shoulder and fanny packs which look like typical hiking gear that work well for quick access to your gun. The best pack to attach to a backpack though is the Wilderness Tactical SafePacker. Google it.
I live in a large retirement community so I deal with these problems a lot. I can check on my website to see if anyone has information that can help you as we discuss gun issues for seniors. If I hear anything I will let you know but in the mean time, if you search you can find what you need.