The movement of oxygen molecules over time from one side of a semipermeable membrane to the other is known as diffusion.

Here's why:

* Semipermeable membrane: This membrane allows some substances to pass through while blocking others. In this case, it allows oxygen molecules to pass through.

* Diffusion: This is the movement of molecules from an area of high concentration to an area of low concentration. Oxygen molecules will move from the side of the membrane with a higher concentration of oxygen to the side with a lower concentration until the concentration is equal on both sides.

Important Note: This process doesn't require energy, making it a passive transport mechanism.