Why `SF_6` is known but `OF_6` is not known

Explanation

Ground state electronic configuration of O(Z = 8) and S (Z = 16) are ,
`O(Z=8) : 1s^2 2s^2 2p^4`
`S(Z=16) : 1s^2 2s^2 2p^6 3s^2 3p^4`
Due to absence of d-subshell in the valence shell of oxygen, it cannot expand its octet and hence cannot show an oxidation state of +6. On the other hand, due to the presence of vacant 3d-subshell in the valence shell of sulphur, it can easily exand its octet and hence can show or oxidation state of +6 in `SF_6`