Oxygen is what we breathe to live, but under pressure too much of it becomes toxic. The relevant number is the partial pressure of oxygen (ppO₂) — the share of pressure contributed by oxygen at depth.
The recreational working limit is 1.4 bar; 1.6 bar is the accepted ceiling for the shallow part of decompression. Above that, the risk of a central-nervous-system oxygen toxicity event (which can cause a convulsion underwater) climbs sharply.
Why nitrox has a shallower MOD
Nitrox has more oxygen than air, so it reaches a dangerous ppO₂ at a shallower depth. That maximum operating depth (MOD) is the trade-off for the longer no-deco time nitrox gives you. Use the Nitrox & gas tool to find the MOD for any mix.