Fuelling the fire[ball]
I don't think we need to look very far, or to unlikely or obscure sources, for the source of the fuel for the fireball; it would have been the kerosene RP-1 propellant.
It would appear that the COPVs used to pressurise the LOX tanks are actually embedded within the tanks but the outer composite wrap was not sealed against penetration by the LOX that surrounded it.
A pressure vessel needs to satisfy two functions: to maintain separation between what's in the vessel and what's outside it, and not to rupture under its rated pressure, and to achieve this Space-X relied upon the impermeability of the aluminium liner to maintain separation and the tensional strength of the outer wrap to withstand and maintain the internal pressure. However, whilst the buckles in the liner may not have significantly reduced the strength of the PV, as that was being provided by the wrap, any buckles in the liner would not have been static as the conditions around it changed; as either the Helium or LOX pressures changed and/or solid LOX formed next to them the buckles in the liner would have been stretched in some areas and compressed in others, eventually resulting in a rupture. As the Helium COPV was at a higher pressure than the LOX, when it ruptured it would have over-pressured the LOX tank in which it was embedded and the resultant failure of the LOX tank would have almost certainly have damaged the rest of the plumbing, including the fuel lines.
Re inflammability of Oxygen: when something burns it is combining with Oxygen in a process that releases heat. The most common example of burning we see in day-to-day life is that of combining Carbon with Oxygen, with rusting probably being the second most common example.