Is it possible that an engine fire in a jet would not be able to sustain itself at cruise altitude due to a lack of oxygen?
If you're asking about a fire insisde the engine than the answer has to do with the engine inlet taking in more air than it flies through.
If you are asking about a fire in the nacelle, than as far as I know its pretty rare at altitude to have anything more than a hot air leak. All the engine nacelle fires with actual flame, that I can recall were either on the ground or at low altitudes.