Electrochemical pile style oxygen sensors continuously deplete themselves whether actively measured or not. Common smart home oxygen piles have a fixed lifetime of a few years, and they're quite sizable (probably about as much volume as a whole smartwatch). Putting the same chemistry in an even smaller package would likely result in lifetime measured in hours

I assume this is because of diffusion of materials at elevated temperature. The sensor would, I think, require a lower temperature than an electrolyzer, since the current would be much lower. But it would be best if lower temperature solid oxide electrolytes could be discovered.