Cylinders are supposed to be pressure tested every few years.
Also they are designed with relief/burst valves that fail at a lower pressure than the tank itself. Tanks are usually secured, so there is little chance that a valve could have been sheared off (which would turn an unsecured bottle into a jet powered missle.)
And most pressurized bottles these days (especially ones used in aviation) are filament wound Kevlar or Spectra (same materials used in bulletproof vests) not steal or aluminum.
A non-metallic tank explosive failure would split the tank, not shoot metal fragments out to cut through the aircraft’s skin, which would have been required to create such a large hole.
A tank rupture might over-pressurize the baggage compartment, causing some minor damage to the structure, but I doubt it would result in that kind of localized hole. I also suspect that there are relief/burst valves in various places of the baggage compartment, that would fail before the skin would. If all of these valves are intact and there isn’t evidence of a sheared tank valve launching the tank through the skin, then logic dictates that this was an explosive device.
In the absence of a fuel, Oxygen isn’t going to explode per se as it is an oxidizer and mixing with the air of the baggage compartment would just increase the Oxygen partial pressure.