[nearbuy]

It is a tricky question, especially if you don't remember the precise definition of combustible/flammable. I believe only the fuel (the thing getting oxidized) in the reaction is considered combustible/flammable, not the oxidizing agent. So oxygen isn't flammable.

[ndriscoll]

I imagine GGP would accept "what we call combustion is an exothermic (hence, fire is hot) redox reaction. Oxygen is a common oxidizing agent (hence, 'oxidation')" as an answer, and the point being made wasn't about the trivia around exact word usage.