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

Explanation

Ground state electronic configuration of O
`O (Z =8) : 1s^2 2s^2 2p^6 `
` S (Z=16) : 1s^2 2s^2 2p^6 3s^2 3p^4`

Due to the absence of d-orbital 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 expand its octet and hence can show or oxidation state of +6 in `SF_6.`