Hypoxia occurs when oxygen supply fails to meet oxygen demand in a specific compartment e.g. tissue hypoxia.
Hypoxaemia refers to a reduced oxygen concentration in the blood due to a reduced partial pressure of oxygen.
Hypoxaemia is one cause of tissue hypoxia (hypoxaemix hypoxia).
Supplementing oxygen can only correct hypoxaemia, if hypoxaemia is not the cause of the hypoxia it will not improve oxygenation.