One of the reasons I replayed Undertale immediately after I finished it the first time was to experience and appreciate the OST.

You can hear 77 of the 101 tracks for free at Toby Fox's official Bandcamp album... but, while I recommend listening to all of them, I actually recommend just buying the game instead (album included or not). The soundtracks' leitmotifs overlap and intertwine with the game's plot line, and the music is so thick with hints and connections between characters/story that, during the first playthrough, you don't even realize how important the music is to the plot well enough to appreciate the attention Toby put into it. He was making sure you were utterly drenched in his actual message, and I think a lot of players were subconsciously afflicted (which many others who played this game don't understand the toxic-looking extent of).

I'm one of the players who went in completely blind of the story, the characters, and the music (and if this is the first time you've heard of this game, I highly recommend you follow my lead). You shouldn't experience the fanbase's afterglow of appreciation for this game until then. If you're emotionally susceptible to music, this game opens a crack in your head to emotions about friendship and love and hope and hate and loneliness you had no clue you were missing or even thought you had to feel in the first place.

I talked it up way too much. Here's Steam store page link if your interest has been piqued: https://store.steampowered.com/app/391540/Undertale/. This probably should be a review there, but I think it's far more helpful here. I feel this community would understand and appreciate what this game is really about.