I guess I was wrong then... Many of you care not for J-culture other than JAV.
But OST is a big deal in Japan. In US/West, even major films (Kill Bill comes to mind) would often select (meticulously) existing songs (sometimes adapted) to make up the sound track, minor productions, especially kid's cartoon would simply use generic stock music and sound effects. Whereas in Japanese, even B- or C- movies or any minor kids anime must commission an entirely original song or two (I'd almost say two is the absolute minimum, an OP (OPening song) and an ED (EnDing song)). This extends to games, dating back to Donkey Kong and Mario Bros, the original theme (no more than a few bars of synthetic MIDI notes) was very important part of a game production.
In Japan, even TV commercials often get OST releases (usually CD-singles, with at least 1 remixed version included). Just saying, compared to the West, original songs are SOP (Standard Operation Procedure).
JAV is, quite curiously, a significant exception. The reason is slightly obvious so let's not dig into it.
Hentai anime, due to its anime heritage, often have original OP (and sometimes ED), but since most hentai are cheap-ass productions, their OST often are cheap-ass forgettable jingles too. Nonetheless the SOP must be followed.
What makes it most interesting is when there's an exception to the exception: when JAV comes with an OP: the ZIZG series of live action version of hentai manga/games. IIRC, the first 2 installments of this series ZIZG-001 Taimanin Yukikaze Live Action and ZIZG-002 Prison Battleship Live Action each have an OP. Let me tell you, no matter how you feel about this genre of JAV, the first few mins of the films is worth watching. Instead of some cheap smut you feel like it's an Akira Kurosawa production but another kind of bodily fluid being spilled.