I don't want to chat with Chlorine Trifluoride, it's nasty.

But yeah, there are some obscure situations where oxygen isn't the oxidizing agent, but the name "oxidizer" gives a clue how rare that is. In most normal situations, oxygen is the oxidizer and the thing it reacts with is the fuel. Partially that's due to Oxygen being a good electron acceptor, but mostly it's because there's a lot of oxygen in the planet, and anywhere you can have humans you pretty much need to have oxygen.