I admit I haven't tried yet because I wanted to ask first.

Both say that you need to own both base games (Skyrim and Oblivion) but you do not need to have them installed.

How does that work? I don't understand how, after downloading the stand alone files, when I go to install them, how it could possibly "know" whether or not I own the games (I do, both). I'm just curious as to how an offline installer can "know" whether or not you "own" a game that isn't installed.

Anyone else wonder this? Anyone know how it does this?

The offline installer does not know, or care, since it's self-contained. GOG, however, does. Which is to say, they know if you bought the game on GOG, but not elsewhere.

It's full stand alone, the game just ask to own Oblivion to add it, not to play it.

Okay, thank you. Marking this as the answer. But, again, this is strictly just curiosity, but I just wonder when the check occurs. As in is it when attempting to "buy" the game(s) (they're both free) it won't let you check out if you don't own the base game or is it when you go to your games library and attempt to download it, there are no download options for you. The latter makes almost no sense so I'm guessing the check is at "purchase" (again, they're free) and anyone not owning the base game(s) can't complete the transaction. That's got to be the way it's done then, right?

Thank you and have a great day.

There are quite a number of games with free DLCs, right? So, while the title is in itself free, you need to own the base game to be able to "buy" it. Well, internally, on the site, those are artificially treated as free DLCs of those respective games and the check is done by the store at the time of purchase. If you don't have the base game, you literally can't get the "DLC", even of it's free, the button on the checkout page is grayed out. Once you are able to get it, there's no further check. Yep, here's how it looks if you try to buy one without having the game.