Lack of oxygen, or hypoxia, can be caused by a wide range of medical and environmental factors. Externally, lack of oxygen is usually the result of exposure to low-oxygen environments, such as those at high altitudes. This is why the FAA recommends that pilots use supplemental oxygen for altitudes over 10,000 feet in the day and 6,000 feet at night, according to About.com.
Hypoxia can be caused by factors that aren't dependent on just the amount of oxygen in the environment, however. About.com gives several examples of medical conditions that can result in poor or inadequate oxygen flow to the brain and other organs. Hypoxic hypoxia describes a condition of low oxygen in the blood that can be caused by inadequate breathing. Anemic hypoxia refers to the blood's reduced capacity for carrying oxygen, which can be caused by many different conditions. Stagnant hypoxia entails poor circulation that starves tissues of oxygen-rich blood. Histiotoxic hypoxia acts on the level of the tissues themselves with cells unable to take up or metabolize oxygen efficiently. Finally, sepsis and other conditions can cause metabolic hypoxia, in which oxygen is taken up and transported efficiently, but the body's need for oxygen rises beyond the capacity of the lungs to supply it.