oxygen does not form OF4 or OF6 whereas S forms SF4, SF6. why ?
The electronic configuration of oxygen is 1s22s22p4. This clearly indicates that oxygen is small in size and does not possess vacant orbitals to accommodate extra electrons. It can only accept two more electrons. Sulphur, on the other hand is large in size and possesses vacant d-orbitals in its valence shell. Therefore it can use its vacant d-orbitals for bonding (covalency) and hence, expand its covalence by forming more bonds.