Right, so as you may know not all of the game's DLC is included in the GotY edition which is typically advertised on Steam. And since there isn't even any way of buying the missing DLC separately from Bethesda anymore, the only way to acquire them legally is to rebuy the whole game as the GotY Deluxe edition. I saw Oblivion listed on today's Steam Winter Sale (again, it's the regular GotY edition with the missing DLC being advertised for some reason). So I decided to just bite the bullet And buy the Deluxe edition for 6,24 euro.

But I already had my previous version installed. And now after my purchase, nothing seems to be any different about Oblivion in my Steam library. How do I get ahold of the rest of the DLC? Or is this something I should talk to Steam support about?

This whole thing has just been really frustrating from the moment I found out my previous Oblivion purchase was incomplete. And now I feel svckered yet again.

Did you actually install the "Deluxe" version after you purchased it?

No, I skipped over that part seeing as I already had the game installed. Then I went to my Steam library and checked the Downloadable Content section for Oblivion, but nothing new was listed there. And now I can't return to the post-purchase page which prompts you to install it.

Go to your library and select the option to view all games. If it's not listed, try verifying the files for Oblivion. If that doesn't do it, you'll probably need to contact Steam support.

Aye, verifying seems to have solved it and downloaded the files. Thanks.