The organ system most directly involved in the transport of oxygen in humans is the circulatory system.

Here's why:

* Lungs: The lungs are responsible for taking in oxygen from the air we breathe.

* Blood: The circulatory system uses blood as its transport medium. Red blood cells, specifically, contain hemoglobin which binds to oxygen in the lungs and carries it throughout the body.

* Heart: The heart pumps the blood, ensuring the oxygen-rich blood reaches all cells in the body.

While other systems like the respiratory system are involved in obtaining oxygen, the circulatory system is the primary mechanism for actually moving oxygen to where it's needed.