I think very few hosts don't oversell but then there is (at least in my opinion) 2 forms of overselling:

firstly a little bit of contention, but not enough to degrade service (usually at the network level) and secondly hyper-overselling where even if everyone used only 5% of their allowed space then the server would simply fall over (albeit due to full disk, maxed io or similar).

We don't oversell anything on our VPS (although there is contention on the network uplink the VPS don't notice any degradation of service due to this). Dedicated servers, what can you oversell? most networks (at least in my experience with UK networks) have more than enough capacity available to allow for a server to use all its monthly transfer and more.

The interesting part arises at shared hosting, on our small shared hosting platform we expect a user to use 75% of their resources but all servers have space for extra hdd and RAM if needed as opposed to maxing the server in terms of disk and ram then piling it high, as this gives us a last resort if a server is using a lot more io or whatever. Obviously then if you're offering reseller accounts then the reseller can start overselling but there's very little you can do there