Oxygen in taken into the lungs through the process of inspiration.
From there, the alveoli in the lungs are able to take the oxygen molecules (O2) and transport them through their thin membranes (diffusion) to capillaries surrounding the alveoli.
The body's cells use the O2 molecules to complete aerobic respiration, of which CO2 and H2O is a byproduct.
The CO2 is taken back to the lungs' capillaries, where it diffuses from the capillaries to the alveoli. This then is ejected out of the body through expiration, and the cycle begins anew.