Oxygen backpacks can be 'transparent' or they can discreetly carry
oxygen portables, like the Freedom Sling an excellent model-
  http://www.freedomsling.com/products/freedom-sling/why-freedom-sling  
well ventilated,over the shoulder centered on the back, but concealed.
    - or
       .
You can do as I have done.  I like my CAIRE constant High flow liquid oxygen portable to be
"naked", so I crafted my own carrier.  Someone asked about it, so here it is out in the open with
just padded straps over the shoulder and a waist belt. It is mounted on to a small backpack
 (concealed behind the portable), the shoulder padded straps and waist strap remained
 in place. Of course the compressed gas oxygen tanks do not need a lot of ventilation,
 my design is more for the very high flow user.
  . 
 The tank - is held in place with three long strips of Velcro ( I had from a sleep apnea test).
 I think most any tank could be velcro'd like this.  Using 12- 15 liters as I do frequently,
 this bare tank approach means much less chance of the tank freezing up.  It did happen
to me once, and I learned to turn the tank upside down for a moment and that helps 'settle
 the liquid oxygen'.  On the front I added a clip on pouch to hold a water bottle,  cell phone,
oximeter, a watch to time duration of walk, so I KNOW when to head home. and of course,
my tape of music from my old 45 records of 1950-60's to keep me more spirited. 

  /  
Note, no cannula, I have a TTO  tube into my neck, transtracheal oxygen.  I 'neck' with oxygen.  
 It is so nice when you can walk about outside... do try to move any which way you can.
Keep moving,
"Lyn"   .