Oblivion's engine I don't think has been used in other games (not talking about DLC or expansions). Same with, pretty much every MMO out there, and how many games are out using The Witcher's engine (again, not expansions), or even Risen, which is supposed to be the same Genome engine from Gothic 3 yet it was almost 'rewritten from scratch'.
I'm talking about taking the same engine, no modifications, and just create new games with it, with a different setting (sci/fi instead of fantasy, or steampunk), different textures, story, etc. That's just not happening.
Well, Oblivion's "engine" is a certain iteration and use of Gambryo - Which was Morrowind before and Fallout 3 and F:NV are now. I don't call this unused.
The Witcher's engine is a different story, though. The underlying script engine was licensed from Bioware, the graphics technology was developed inhouse. I think that there is not much documentation there to sell the tech to other companies, especially because noone was keen on the engine - I remember reviewers calling the graphics "dated".
And AFAIK Risen does not use Genome, but I may be wrong.
If you want something like this, then you'll have to leave it to the community. Look at NWN or The Witcher - there is a lot of MOD material that count as fully new games…