Oxygen, as we know it, doesn't come from a single molecule. It's produced through a process called photosynthesis.

Here's the breakdown:

* Photosynthesis: Plants, algae, and some bacteria use sunlight, water, and carbon dioxide to create their own food (glucose) and release oxygen as a byproduct.

* Water (H₂O): The oxygen atoms in the oxygen molecule (O₂) come from the water molecules used in photosynthesis.

* Carbon Dioxide (CO₂): The carbon dioxide molecules are used to create the glucose, but their oxygen atoms are not used to form the oxygen gas released.

So, in essence, oxygen comes from the splitting of water molecules during photosynthesis.