(By OST I mean backround music to a show/game/ect)
Mine is a tie between:
Cascade (Flare more specifically) from Homestuck. It is a 13:14 long combination of at least 4 other songs and it is epic, half since a million plot advancing things are happening right after one another in a 13 minute long flash, and another since the song is just so good. (I'd explain more but the whole thing is a huge spoiler don't watch videos of it seriously)
Puella in Somnio/The Girl in the Dream from Puella Magi Madoka Magica. This is the theme of one of the main characters, Homura Akemi. At first you just think it's a pretty cool song. Once you figure out whats really going on, the hollow feel of the notes start to make sense. For that reason, I consider it one of the best themes I've ever herd.