Why is an oxygen molecule more stable than and oxygen atom?

Oxygen molecule is 2 oxygen atoms in a covalent bond, whereas an oxygen atom is an atom with 8 electrons (6 valence electrons).

Thus, clearly, any element in a bonded state is always more stable than when in an un-bonded state.

Both atoms in an O2 molecule have achieved a noble gas electron configuration. - An oxygen atom does not have a stable octet of valence electrons.