One of the differences between O2 (oxygen molecule) and N2 (nitrogen molecule) is the bonding. O2 has a double bond and N2 is a triple bond (these are both covalent bonds). This means that the bond in N2 is stronger than in O2. Which makes sense as we know that N2 is a highly stable (unreactive gas) whereas O2 is much more reactive.
Interestingly enough even though people often think of the air we breathe as being mostly O2 it’s actually composed of about 78% N2, 21% O2 and about 1% other gases like Argon (Ar) and carbon dioxide (CO2) . Our bodies rely on this composition (or at least something close to it, we can actually tolerate pretty low concentrations of O2 at high altitudes for examples) and if the air was made up of 100% O2 it would actually be deadly to humans. It would throw off our respiratory (breathing) system and our lungs would fill up with fluid.