Well I would suspect that part of the o2 tanks you see is that you're in Montana. When I lived in Colorado (Boulder), some of the serious riders I saw carried o2 as well ... when you're bike riding at a mile up, the oxygen is a little thin. If you've ever watched a football game played at Mile High Stadium in Denver, it's the same thing - they keep oxygen on hand for the players - especially the players from out of town who aren't used to working in a thinner atmosphere.
Your muscles fatigue faster if you're working with less oxygen than your body is used to and so serious athletes who are working at altitude do, often, carry extra o2 for that purpose.