The absence of which orbitals in oxygen limits its covalency to four and in practice, rarely exceeds two:
p-orbitals
d-orbitals
f-orbitals
None of these
Due to its small size and high electronegativity, it shows some anomalous behaviour, like other members of p-block present in the second period.The absence of d orbitals in oxygen limits its covalency to four and in practice, rarely exceeds two while the valence shell of other elements can be expanded and covalence exceeds four.