I am trying to establish where the Oblivion install creates its Windows Registry information for various operating systems.
It's going to be in the same place on all operating systems, with the only complication being the Wow6432Node part. If you're installing on a 64-bit OS, then it'll be there. However, if you're running a 32-bit app, then the registry will be virtualized such that you won't see it. So:
64-bit OS, 64-bit app: HKLM\SOFTWARE\Wow6432Node\Bethesda Softworks\Oblivion
Anything else: HKLM\SOFTWARE\Bethesda Softworks\Oblivion
To try this out, run the copy of regedit from c:\windows\syswow64\ and compare with the results from the one in c:\windows\system32\. (yes, it's backwards: the one in syswow64 is the 32-bit version and the one in system32 is 64-bit)