Anomalous behaviour of oxygen is due to its

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 ${O}_{2}$ 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 $p \pi - p \pi$ multiple bonds.