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.