Oxygen is required for aerobic respiration. This reaction produces ATP (universal currency for energy) which our bodies use to survive. Without oxygen much less ATP is produced, as aerobic respiration cannot occur in the lack of oxygen. ATP can be formed by anaerobic respiration, in the lack of oxygen, but this produces far far fewer molecules of ATP. This also produces lactic acid causing us to become fatigued and tired. Thus oxygen is crucial to produce the maximum amount of ATP possible.
Glucose is the main molecule which is respired by our bodies to produce ATP/ energy, which our bodies use to survive. Oxygen and glucose react in the mitochondria to make ATP (release energy.) It therefore, is called a respiratory substrate.
Note: I'm avoided talking about the exact phases of respiration as it is usually not covered till college/ A level standards. If you would like to know exactly which reaction requires oxygen and how it's used (i.e. the stages of respiration) don't hesitate to ask.
Photosynthesis is energy to sugar, and cellullar respiration is sugar to energy.
