Official OST

Season 1:

Opening: Cowboy Bebop - Rush

https://youtu.be/l8wWa3O9cUo

Ending: Hunter x Hunter - Reason (Nightcore)

https://youtu.be/v8Ee771P4cI

Harvey's main theme: JoJo's Bizarre Adventure - Dark Rebirth

https://www.youtube.com/watch?v=_Qq1B5na--s

Harvey's secondary theme: FNAF Ambience

https://www.youtube.com/watch?v=fJIhjGSD3-0&t=385s

Harvey's battle theme (when he's shredding notes on clone hero): Touhou FDF - Who Cares

https://www.youtube.com/watch?v=kTFt-wsRMys&t=1025s

Theme of rejection 1 (anger): Bakemonogatari - Sensou

https://youtu.be/-d69ldAGPn0

Theme of rejection 2 (depression): Hunter x Hunter - Requiem Aranea

https://www.youtube.com/watch?v=8hFb10p7uWg&t=2s

Arch-rival YandereDev's theme: Berserk - My Brother

https://www.youtube.com/watch?v=lBdnti4_UUg

Harvey's theme of power: JoJo's Bizarre Adventure - Awaken

https://www.youtube.com/watch?v=XUhVCoTsBaM

Harvey at the end of every episode when hes crying himself to sleep in the dark confines of his room: FMA:B - Trisha's Lullaby

https://www.youtube.com/watch?v=l2kwie-gs2o

Music that plays in the final scene as harvey snaps and commits an atrocity by unleashing his rage upon his college in an act of violence and arson: DOOM 2016 - BFG Division

https://www.youtube.com/watch?v=pNkQMtZAMAw

 Season 2: 

Opening: Corpse Party - Chapter 1 Theme

https://www.youtube.com/watch?v=h0ytFDJjKmg