I don't have any OST that made me cry, but if its okay, I'll post an AMV of Nostalgia. The song is A Thousand Years by Christina Perri. This song really suits their story. I cried so hard I was sobbing for the next couple of hours. ^^
XD Its because This Anime is from the same Author of Junjou Romantica, so art is quite alike. This Anime is Sekai Ichi Hatsukoi, its as amazing as Junjou Romantica! Do watch it! ^^