What does it mean if you have low oxygen levels in your blood tests?

Lung disease. Is the commonest reason for a low level of oxygen in the blood. It can also happen in heart disease if the heart is failing and fluid builds up in otherwise-healthy lungs, or if there is a hole in the heart, causing blood to bypass the right heart circulation into the lungs and skip picking up oxygen there. These are examples of some common causes. Smoking, high altitude=examples of lung reasons.