Here's why:

* Concentration gradient: Molecules move from an area of high concentration to an area of low concentration.

* Directly through the cell membrane: This means no additional energy or proteins are required to facilitate the movement.

* Simple diffusion: This is the passive movement of molecules across a membrane from high to low concentration without the assistance of any membrane proteins.

Oxygen, being a small, non-polar molecule, can easily pass through the lipid bilayer of the cell membrane via simple diffusion.