There are two ways you have to earn Oliver's spells:
1. Progress through the story.
2. Solve riddles that are given to you by Horace.
If you need help solving the riddles here are the answers (English version only)
*** Spoiler - click to reveal *** To find Horace in these locations you have to solve the riddles in this order:
1. Location: Ding Dong Dell - Stood behind a stone monument.
Riddle answer: No riddle the first time you meet him.
Spell: Spirit Medium
2. Location: Al Mamoon - Stood near the entrance to the palace next to one of the fountains.
Riddle answer: Dragon Of The West
Spell: Draw Poison
3. Location: Castaway Cove - Stood on the bridge.
Riddle answer: Finest Fiber
Spell: Vacate
4. Location: The Fairyground - On the path to the Fairy Godmother.
Riddle answer: Crispy Lettuce
Spell: No spell, he will give you the alchemy formula for Sage's Secret instead.
5. Location: Hamelin - On the balcony, overlooking the main street.
Riddle answer: 5
Spell: Chart Chest
6. Location: Yule - In the Elder's house.
Riddle answer: Blizzard Bloom
Spell: Cloudburst
7. Location: Perdida - Inside Swift Solutions.
Riddle answer: Jabal al-dukhan or Old Smoky
Spell: Veil
8. Location: Ding Dong Dell - Behind the stone monument where you first met him.
Riddle answer: Unleash
Spell: Thunderstorm
9. Ara Memoriae - At the ruins but only after you beat Gallus.
Riddle answer: Wizard's Robes
Spell: Astra