* Chemical compounds are formed when two or more different elements combine chemically in a fixed ratio.

Here's the analysis of the provided options:

* O2 (Oxygen): This is a diatomic element, meaning it's made up of two atoms of the same element (oxygen). It's not a compound.

* H2O (Water): This is a compound because it consists of two different elements, hydrogen (H) and oxygen (O), combined in a fixed ratio (2:1).

* CO2 (Carbon Dioxide): This is a compound because it's made of carbon (C) and oxygen (O) in a fixed ratio (1:2).

* N2 (Nitrogen): Similar to O2, this is a diatomic element, not a compound.

Therefore, the chemical compounds are H2O and CO2.