The covalent bond in diatomic oxygen is a double bond and is stronger than the single covalent bond in diatomic chlorine.
Covalent bond
Yes, the molecule of oxygen (and the molecules of other gaseous elements) is diatomic; but ozone contain 3 oxygen atoms.
The chlorine atoms share two electrons between themselves, while the oxygen atoms share four. This can alternatively be stated as a single bond between the two chlorine atoms and a double bond...