Oxygen does not dissolve directly into blood under high pressure existing in the deep sea, hence inert gas like helium is mixed with oxygen during deep sea diving.
Breathing gas or popularly known as Trimix consist of oxygen, helium and nitrogen which is used in Scuba tanks used in deep diving, and recreational diving.
Sometimes a mixture of helium and oxygen with a 0% nitrogen content is used. This gas is known as Heliox. This is frequently used as a breathing gas in deep commercial diving operations.