Here is the full list of all 13 Oh Darling! I Hate To Tell You This... achievements.
Beat the Holy Orb Boss
Beat all the Big Bosses
Beat Your Inner Demon
Choose all the Bullet types
Choose the Exploding Bullet
Kill 666 Demons
Choose the Big Bullet
Complete the sentence
Beat the Mom Boss
Beat the Lady
Kill 120 Demons
Beat the Cyclops
Choose the Double Bullet