Oxygen enters the body through nostrils, then it passes through the nasal passage and come in the pharynx. Now the oxygen travels into trachea via larynx. Trachea gets divided into left and right bronchi which further gets divided into bronchioles. Bronchioles get terminated into alveolar sacs.

Nostrils → nasal passage → pharynx → larynx → trachea → bronchi → bronchioles → alveoli