First, open the CS with just Oblivion.esm checked.
Select World from the menu bar.
Select World Spaces...
Scroll down to OblivionRD001
Right Click on that or one of the other OBlivionRD Worlds
Select Duplicate.
Rename the duplicate to something along the line of XXORP, where xx are your initials.
Click OK.
Save your esp with whatever name.
Close the CS.
Open your new esp with TES4Edit.
Type 00091C9C in the FormID field, press enter.
(That is the FormID for the Oblivion Gate that will teleport you to the random realms).
Right click on the FormID and select "Copy as override into..."
Click Yes at the warning.
Click the check box for your new esp.
Click OK.
On the right side of TES4Edit, you should see the "Door" from Oblivion.esm added to the new esp.
Under the new esp, right click the empty field across from "Random Teleport Destination" and select Add.
Now there should be a "Null Reference" at the bottom.
On the Left side, click on the + next to your new esp.
Click on the + next to Worldspace.
There's the FormID for your new realm.
Write that number down.
Back on the right side, right click on the "Null Reference" field and select Edit.
Type in the FormID of your new worldspace and click OK.
(At this point, for testing purposes, I deleted the 7 default worlds from my esp so every gate but the city gates will teleport to only my worldspace. The finished esp from InsanitySorrow will include the default worlds, I beleive.)
Close Tes4Edit, Save the esp.
Done.
Now you have a base worldspace for an Oblivion Realm.
Open the CS and have at it.
Aellis before you send me your esp, will you add in those 7 OB realms back into the list with yours?, saves me some TES4 Edit work .
Its only one door so far in the CS that actually links to the realms, so all new realms will be added to that list, which includes the OB ones too.