Here is the full list of all 30 Ongaku achievements.
Complete all beginner levels.
Complete all easy levels.
Complete all medium levels.
Complete all hard levels.
Complete all very hard levels.
Achieve Gold Ribbons on all beginner levels.
Achieve Gold Ribbons on all easy levels.
Achieve Gold Ribbons on all medium levels.
Achieve Gold Ribbons on all hard levels.
Achieve Gold Ribbons on all very hard levels.
Achieve S Rank on all beginner levels.
Achieve S Rank on all easy levels.
Achieve S Rank on all medium levels.
Achieve S Rank on all hard levels.
Achieve S Rank on all very hard levels.
Achieve a 100 hit Crescendo.
Achieve a 200 hit Crescendo.
Achieve a 300 hit Crescendo.
Achieve a 400 hit Crescendo.
Achieve a 500 hit Crescendo.
Start the first Ongaku game.
Move Ongaku for the first time.
Scoop or Hit a Paint Bubble.
Power up Ongaku to Ultra state.
Power up the stream to maximum.
Get all O.N.G.A.K.U. Bubbles for the first time.
Hit 10000 Paint Bubbles Perfectly.
Unlock the Neven Expanse.
Watch all the Credits.
Create your own level.