Oxygen

Oxygen is an chemical element.

Oxygen can be toxic when concentrated. There are both long and short term effects; you should be mostly immune to short term effects at that concentration (see also: medical oxygen therapy). Long term, oxygen oxidizes stuff, and that can be bad (see also: antioxidants are good for you). You'll probably see some dna damage related stuff like more cancer, but I don't think it would be an immediate problem unless you had compounding health problems.

You're wrong. Although the free radical theory of aging is popular, and cells cultured in high O2 sometimes enter replicative senescence faster, there is no evidence to suggest that exposing organisms to higher O2 concentrations directly leads to them aging faster. In fact, lab-kept mole rats thrive in atmospheric oxygen despite the fact that they evolved to live in low oxygen subterranean burrows. Source: am a professional biogerontologist.