We don't just look for oxygen. For instance, there was some excitement a few years back when there was a tentative observation of phosphine in Venus's atmospher

We don't just look for oxygen. For instance, there was some excitement a few years back when there was a tentative observation of phosphine in Venus's atmosphere. To have a good biosignature, you want a signal that cannot be explained by anything other than life. Of course, that means that there might be planets with life that we do not notice, because all of the observables we have access to have an abiotic explanation. Not much you can do about that other than a bunch of science.