The oxygen atoms in O2 produced during photosynthesis come from water (H2O).

Here's a breakdown of the process:

1. Light-dependent reactions: Sunlight is absorbed by chlorophyll in the chloroplasts of plant cells. This energy is used to split water molecules (H2O) into hydrogen ions (H+), electrons, and oxygen gas (O2).

2. Oxygen release: The oxygen atoms released from the water molecules combine to form O2, which is then released as a byproduct into the atmosphere.

Essentially, the oxygen we breathe is a waste product of photosynthesis.