How much oxygen breathed in is breathed out?

Actually, we breathe in oxygen from the air which is furnished by green plants, and when the oxygen cycles through our system, it is converted to carbon dioxide which is what plants need. It is like...

if oxegyen was water then about 5 gallons of water you breath in each breath

Strictly, not. There is an oxygen gradient in pulmonary alveolus that permits access to blood, which depends of the ventilation/perfussion relationship. Besides, in the dead space (mouth, traquea,...

The red blood cells pick it up from the air and take it through the blood stream to the cells in the body.

Pure oxygen can be breathed. Pure oxygen is often given to patients in hospital that have low O2 saturation.