An explosion is a sudden increase in volume and release of energy
"An explosion is a sudden increase in volume and release of energy in a violent manner, usually with the generation of high temperatures and the release of gases."
Of course there is no chemical reaction producing the rapid expansion of gas and release of energy when an O2 tank goes. It's still an explosion. A heated propane tank explodes, and then burns. A nuclear weapon explodes due to the sudden release of energy. The nuclear material doesn't interact with O2 to produce all that rapid expansion.
Arf