I've used a few multigas sensors that had O2 when mapping volcanic caves -- Draeger Multiwarn, MultiRAE, National... Safety (?), and CrowCon GasMan. The CrowCon was my favorite by far. Disclosure: they gave us equipment for free as sponsorship for our expedition, but it was also honestly just way better than the Draeger or RAE. Great datalogger built-in with good serial interface, small, waterproof enough for our uses (the others said IP68 and still died of water damage). I suspect all CrowCon products are quite good and cheaper than the competition.

Or, if you're building your own unit, then I remember citytech (welcome to city tech!) was pretty much the only option when I was doing it. The circuits to run electrochemical gas sensors are a little annoying to set up.