The last two electrons go into separate, degenerate π orbitals, according to Hund's Rule. Thus, oxygen has two unpaired electrons and is paramagnetic.

Is O2 − paramagnetic or diamagnetic?

Oxygen is paramagnetic mainly because it consists of two unpaired electrons in its last molecular orbital.

Is O2 molecule diamagnetic?

O2​ molecule is diamagnetic while C2​ molecule is paramagnetic in nature.