I’ve been humming the little piano motif from 'Thirty But Seventeen' all week, and honestly it makes me want to rewatch the whole thing just for the music. If you want the complete official tracklist, the quickest trick I use is to search the Korean title '서른이지만 열일곱입니다 OST' on streaming services — Spotify, Apple Music, YouTube Music, or Korean sites like Melon and Genie. K-drama OSTs are usually released as Parts (Part.1, Part.2, etc.) as singles first, then compiled into a full OST album, so you’ll often see both the episodic releases and the combined album listed separately.
On those platforms you’ll find the vocal tracks (the main ballads and duet pieces that played during pivotal scenes) plus the instrumental BGM pieces (piano themes, strings, and character motifs). If you prefer a one-click method, YouTube playlists titled 'Thirty But Seventeen OST' will gather everything including instrumental versions and sometimes fan-subbed lyric videos. That’s how I cross-check titles and artist names before I create my own playlist, and it works every time — also helps to spot covers and live versions that fans upload.