...that the only way the pressure could rise in the oxygen tank is if
1) More oxygen were added
2) The oxygen warmed up
3) The tank got smaller
The first is not possible. The second seems unlikely; a fire near oxygen normally would result in some sort of explosion, not a gentle warming up. An explosion would have destroyed everything, yet the first stage was clearly still operating after the first stage had got into difficulties. So it might have to be the third.
That implies that there was a structural failure of the upper stage resulting in the oxygen tank crumpling up as the stage collapsed lengthwise.
If it is a structural collapse, that's pretty bad. That's the most basic and easiest part of rocket science. It would be most unfortunate if this were indeed the cause of the failure. Some panel falling off from higher up could result in a puncture, which I suppose might weaken the stage structure. There did appear to be quite a lot of gas streaming off the upper stages during the ascent.
As Mark85 quoted, "The rocket science is the easy part, the doing is the hard part.". A launch failure due to a structural failure obviously won't help convince any future astronaut that it has been built right, never mind designed right. Safe flight operations is largely about having the right process and sticking to it; SpaceX's process clearly still has some flaws which need to ironed out.
As the great Feynman said:
"For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled."
I'm hoping that Elon Musk has that put up on a large poster opposite his desk in his office, and everyone else's too. NASA were serially guilty of putting PR first, and it cost Astronauts' lives.
Anyway, I'm sure they'll find the problem and get it ironed out. I hope so. If there's one good thing to come out of this, it should be the question, "What else have we missed?".