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 por Christina Perri. This song really suits their story. I cried so hard I was sobbing for the seguinte couple of hours. ^^
XD Its because This animê is from the same Author of Junjou Romantica, so art is quite alike. This animê is Sekai Ichi Hatsukoi, its as amazing as Junjou Romantica! Do watch it! ^^