Due to its small size, Oxygen exhibits high electronegativity, a high charge/ radius ratio, and no d-orbital oxygen shows anomalous behaviour.
Oxygen exists as a diatomic molecule as O2 comprising a double bond whereas all the other elements of group 16 exist as polyatomic molecules.
Oxygen is paramagnetic and exists in a gaseous state at room temperature, which is completely opposite of the other Group 16 elements.
The small size and high electronegativity of Oxygen enable it in the formation of pπ−pπ multiple bonds.