Respiration- Oxygen is life-supporting gas, which is used by animals to oxidize food during respiration and get the energy to live.
For sea diving and mountain climbing- Sea divers and mountain climbers carry oxygen with them as the density of air decreases with an increase in altitude or on going deep into the sea.
For medical purposes- In hospitals, oxygen is given to patients suffering from respiratory ailments for artificial respiration.
In space rockets- Liquid oxygen is used to burn fuel in space rockets and missiles.