The Self-Contained Underwater Breathing Apparatus (SCUBA) does not contain pure oxygen but air!
Note that atmospheric air as used for breathing acts like a two-component gas mixture with

a main component (Nitrogen) that is chemically inert but becomes narcotic under pressure, and

a chemically active component (Oxygen) that becomes toxic under pressure (at 2 or three Bar partial pressure, but fortunately it is considerably diluted by the nitrogen).

Another chemically inert gas (Argon) that is even more strongly narcotic than nitrogen is however only present in low concentration, so that its physiological effects are irrelevant in practise.
In expired air, the oxygen concentration drops markedly, and several percent of CO2 appear, that is again chemically active (i.e. narcotic and toxic). Its persistently low level in atmospheric air is due to the existence of sinks (plant metabolism, sea water, carbonate rocks).