The bottle you will breathe first goes on the "outside." It needs to be there to facilitate your budding checking your breathing gas MOD, etc. Since you are asking this question, I am going to assume you don't have any need to deal with more than two bottles at this time. That means if it was a 70' bottle and an O2 bottle, you would place the O2 bottle to the inside.
What they teach you in the tech classes is what matters for assuring you are breathing the right mix, not the order of where bottles are stored. Once it goes to three or more bottles, my primary considerations for where to place bottles are my comfort at deco (one bottle on left most of the time particularly if not al40s) and the easist and quickest way of switchiing bottles to the left d-ring leash after use.