Answer:
Oxygen goes in through the mouth/nose, down pharynx, down larynx, through the trachea, into the lung, into the bronchus, into the bronchioles, and then finally to the alveoli.
Alveoli are little sacs that have tons of surface area on them in order to make diffusion of oxygen and carbon dioxide easier. The fresh oxygen diffuses into blood cells while carbon dioxide is diffused out. This carbon dioxide is released as an exhale.
Then, the oxygenated blood cells travel to the heart where they are pumped to the rest of the body, depositing oxygen to every blood cell. When all of their oxygen is gone, the cycle repeats over and over again.