Good afternoon,

Do you have to own the base game to buy the music or can I just buy the music from games and download those?

In most cases they are considered dlc, and you can’t buy dlc without owning the base game. End of the day you can try it, it just won’t let you pay for the order.

I believe as long as the OST is separately purchasable, you can buy that without owning the game.

It depends on the game:

-Some games include the Soundtrack as a bonus goodie you can download from the extras section on the game page.

- Some games offer it as DLC were you need the base game to purchase.

-Some games offer the soundtrack as a stand alone download.

So it just comes down to what game you would like the soundtrack for and if that games offers the soundtrack as a stand alone item, offers it as bonus content, or doesn't have a soundtrack available at all.

Out of all products on GOG that have "soundtrack" or "OST" in the title (I'm not putting in more effort to locate any stray products which don't fit the pattern), only these can be purchased independently as of this writing:

Lost Ember - Original Game Soundtrack
Lovely Planet OST
The Pedestrian Soundtrack
Owlboy Soundtrack
Obduction Soundtrack
My Friend Pedro Soundtrack
X4: Split Vendetta Soundtrack

You can search for them by hiding DLCs in the filter menu and seeing what remains.