I still have my 360, because not all the games I own are available on xbox one's backwards compatible list.
Im going to try doing it again and see if it registers in game. So how I got it on my xbox one was, on my 360 I uploaded the saved dlc to cloud saves. To go to the xbox one "ready to install" in order to download it. It shows up as purchased on the in game dlc list but tells me Even though it's one of the 2 I already owned on that account. Dan Bull Take Me To Oblivion: This is a letter to Bethesda In which I beg you to stick together For. So I deleted it and was going to go re install, only to see that it's not in my available "ready to install" section. Wizards tower showed installed in the xbox one, manage game menu but not the in game menu Mehrunes razor still said I didnt own it so I had to pay for it again (3rd time) to make it work.
So now I decided to just buy it all again because I couldn't (Possible solution at the bottom) Years ago, before skyrim, I split dlc purchases for oblivion between 2 different accounts on my 360 with only 2 being on my main account.