So... why the heck does Octillery lack Shadow Ball?
Is strange, Shadow Ball is one of those TMs they like to give out like candy. Also Octillery can learn Energy Ball, the (former) Grass-type Shadow Ball. It's either an oversight or, with how much coverage it already gets, they figured that Shadow Ball either doesn't really help Octillery much or its to give some Types (namely Psychic and Ghost) a more guaranteed safer match-up as Octillery's other options to deal with them aren't as good as Shadow Ball. If we're to look at what moves Octillery has to handle every type:
Normal: None
Fighting: Psychic, Bounce
Flying: Ice Beam, Blizzard, Charge Beam, Rock Blast
Poison: Psychic, Mud Shot
Ground: Surf, Hydro Pump, Ice Beam, Blizzard, Energy Ball
Rock: Surf, Hydro Pump, Energy Ball, Flash Cannon
Bug: Flamethrower, Fire Blast, Rock Blast, Bounce
Ghost: Thief, Payback
Steel: Flamethrower, Fire Blast, Mud Shot
Grass: Ice Beam, Blizzard, Flamethrower, Fire Blast, Signal Beam, Bounce, Sludge Wave, Sludge Bomb
Fire: Surf, Hydro Pump, Rock Blast, Mud Shot
Water: Energy Ball, Charge Beam
Electric: Mud Shot
Psychic: Signal Beam, Thief, Payback
Ice: Flamethrower, Fire Blast, Rock Blast, Flash Cannon
Dragon: Ice Beam, Blizzard
Dark: Signal Beam
Fairy: Sludge Wave, Sludge Bomb
While some options are better than others, Octillery is no one trick pony when it comes to coverage. Honestly it needs Focus Blast (or Aura Sphere) more than Shadow Ball so it can claim having a move to do super effective damage to every single type.