It's the age old debate. I think the thermal det thing goes back to sometime in the 90's when the Star Wars Visual Dictionary came out and the o2 tank was labeled as a thermal det. I want to say there was another book from the late 80's that also listed it as a thermal det, but it's been too long and I just don't remember. I think in a way the new ep 7 Stormtrooper armor supports the o2 tank theory, since it looks like it wraps around the tank and wouldn't be easy to remove if it was ment to be a thermal det. Anyways it is just one of those things that flipflops every few years. It'll get mentioned in one book as a thermal det, another as an o2 tank. I use think it was a thermal det for years until some of my buddies that are 501st members educated me about it.
Until Lucasflim/Disney makes an offical statement about it just call it which ever you prefer. Because there are offical sources that support both options.