Priestley originally called oxygen depholigisticated air. The atmosphere gives us oxygen to breathe, and it serves as a blanket to keep us from freezing or burning. Plants give off oxygen. When people breathe oxygen, it goes into their lungs. Then it travels into the capillaries that are filled with blood, then into the cells.
Rocket engines use oxygen in many ways.
They use it for the fuel and for the passengers to breathe. You have heard that fuels need oxygen in order to burn. Yet only a part of the air is oxygen. We should be thankful that most of the air is nitrogen, a colorless gas the quenches fires. Also in 1774 Priestley became the first person to make a pure ammonium gas.