These O2 tanks can be heavy - depending on the size bottle she has. My Mom was on O2 and the really small bottle which she could easily carry in her hand only lasted her a short journey while the larger bottles were hard for her. I found this site
http://maidenjane.blogspot.com/2013/...-and-yoga.html
which had a cute design - and the straps were long enough to carry over the shoulder. Good luck - I do remember how important it was to her to be able to get out and about. An other thought is to create a really cute "backpack" type if she could tolerate carrying it on her back. Good luck with your design