Oxygen by itself doesn't burn. If it was just the LOX tank that burst, there wouldn't have been anything to burn. Clearly some sort of FTS destroyed the vehicle (I don't think atmospheric forces could have taken it apart that cleanly and instantaneously), but the FTS system is probably just detcord running down the length of the propellant tanks. If the tanks were 'unzipped' quickly enough, the fuel and oxygen might not have had time to mix to a stoichiometric ratio that supported combustion.
Looking back at a gif of the incident, there was a pretty impressive amount of fire. Initially when the oxygen hit the first stage exhaust, causing the unburnt RP1 to burn (all the 'extra' flame that appears in the exhaust plume), and then there is a fair amount of fire at the front of the vehicle a second or two later (I assume the second stage RP1 tank failed at that point). Finally there is a large cloud and the rocket vanishes. That's the part that I assume is some sort of FTS, and likely blew the rocket apart with enough force that the prop didn't get a chance to mix.
