# Why can sulfur have an expanded octet but not oxygen?

In my search to understand the bonding in structures like $$\ce{SF6}$$ I found many sources that said it was because sulfur has d orbitals to accommodate an expanded octet, which made sense to me. But I also found sources like the paper by Reed and Weinhold (1986) that say d orbitals contribute very little to the bonding in $$\ce{SF6}$$. An alternative explanation is that $$\ce{SF6}$$ has three-center/two-electron bonds. What I don't understand is why compounds like $$\ce{SF6}$$, $$\ce{ClF5}$$, $$\ce{PCl5}$$ exist for third row element centers, but not $$\ce{OF6}$$ if it has nothing to do with d orbitals? References would be great.