"Oxygen molecules" refers to very tiny particles that make up the air we breathe. These molecules are essential for living things to survive because they help our bodies produce energy.
Otherwise, it would be difficult to tell what the «normal» amount of circulating reactive oxygen molecules in the animal's body was. (sciencedaily.com)
The new cobalt catalyst also splits water into hydrogen and oxygen without creating the kind of reactive oxygen molecules that can damage DNA or other processes essential to continuing life. (scientificamerican.com)
They estimate this temperature based on measurements of the microwave emissions of oxygen molecules in the atmosphere, which increase proportional to temperature. (yaleclimateconnections.org)