Here's my ORAS OU Singles team. It seems to work pretty well, but it can use some tweaking.
Starmie @ Leftovers
Ability: Natural Cure
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
- Surf
- Rapid Spin
- Ice Beam
- Psychic
Starmie is my star special sweeper. Surf, Ice Beam, and Psychic for coverage and power. Surf and Phychci for coverage, and Ice Beam for Landlorus and Garchomp. Rapid spin is to deal with entry hazards. I don't want to use Defog since I use some as well and I don't want to remove them. EVs and nature are to maximize sp. attk and speed.
Ferrothorn @ Rocky Helmet
Ability: Iron Barbs
EVs: 248 HP / 8 Atk / 252 Def
Impish Nature
- Stealth Rock
- Knock Off
- Leech Seed
- Seed Bomb
This guy has been a thorn in everyone's side since it was introduced. Stealth Rock to chip away at HP, especially Talonflame and Charizard. Knock Off is always fun to remove the foe's item and make some damage. Leech Seed is fun to drain HP and to encourage switching out for a Dragon Dance Garchonp or Quiver Dance Volcarona. Seed Bomb is for some STAB power, and to deal with any water/ground/rock types. Rocky Helmet and Iron Barbs are combined to hurt anyone who wants to touch Ferrothorn. EVs and Nature are to add to defense and HP to help the iron barbs/rocky helmet combo.
Gyarados @ Persim Berry
Ability: Intimidate
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Earthquake
- Dragon Dance
- Waterfall
- Outrage
Who doesn't love this guy? A staple to competitive battling since Gen I and I can sea why. Dragon Dance sets up Gyarados to be a strong sweeper. Earthquake, Waterfall, and Outrage are all picked for their power and coverage. Waterfall is picked over Aqua Tail for the better accuracy. Ice Fang is left out, but a good addition to this guy. Persim Beryy is there to help with the confusion after Outrage. Intimidate is always fun to run, and EVs and nature are to help sweeping.
Staraptor @ Leftovers
Ability: Intimidate
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Brave Bird
- Close Combat
- Quick Attack
- Pursuit
I feel that this bird is always a underrated, but it can make anyone bird-or-prey for their lives. Brave Bird is great for it's power combined with Staraptor's grat attack stat. Close Combat helps with anyone who would normally wall Staraptor, like Steel and Rock types. Quick attack for STAB and speed priority, and Pursuit for when they run from Staraptor. Leftovers help restore HP from Brave Bird's recoil, and Nature and EVs help with sweeping. And as previously stated, Intimidate is just fun.
Altaria @ Altarianite
Ability: Natural Cure
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Earthquake
- Return
- Heal Bell
- Dragon Dance
Another Dragon Dancer. Dragon Dance set's up Earthquake and Return, and once it Mega Evolves, Return gets Pixelated and turns into a Fairy type move, making the combo great coverage. It can really Fluff you up. Heal Bell is to take advantage of M-Altaria's bulk and add some support to the team. EVs and Nature are to add speed and power.
Klefki @ Leftovers
Ability: Prankster
EVs: 252 HP / 252 Def / 4 SpA
Bold Nature
- Thunder Wave
- Spikes
- Magic Coat
- Flash Cannon
The Key to this team. Thunder wave and Spikes are to slow down and hurt the opponent. Magic Coat is bounce anything odd back like stealth rock, will-o-wisp, or a stray toxic. Flash Cannon is to add a little offense and to avoid being taunt bait and to hurt any fellow fairy types. Leftovers is for recovery, and Evs and Nature are to add some bulk.
Starmie @ Leftovers
Ability: Natural Cure
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
- Surf
- Rapid Spin
- Ice Beam
- Psychic
Starmie is my star special sweeper. Surf, Ice Beam, and Psychic for coverage and power. Surf and Phychci for coverage, and Ice Beam for Landlorus and Garchomp. Rapid spin is to deal with entry hazards. I don't want to use Defog since I use some as well and I don't want to remove them. EVs and nature are to maximize sp. attk and speed.
Ferrothorn @ Rocky Helmet
Ability: Iron Barbs
EVs: 248 HP / 8 Atk / 252 Def
Impish Nature
- Stealth Rock
- Knock Off
- Leech Seed
- Seed Bomb
This guy has been a thorn in everyone's side since it was introduced. Stealth Rock to chip away at HP, especially Talonflame and Charizard. Knock Off is always fun to remove the foe's item and make some damage. Leech Seed is fun to drain HP and to encourage switching out for a Dragon Dance Garchonp or Quiver Dance Volcarona. Seed Bomb is for some STAB power, and to deal with any water/ground/rock types. Rocky Helmet and Iron Barbs are combined to hurt anyone who wants to touch Ferrothorn. EVs and Nature are to add to defense and HP to help the iron barbs/rocky helmet combo.
Gyarados @ Persim Berry
Ability: Intimidate
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Earthquake
- Dragon Dance
- Waterfall
- Outrage
Who doesn't love this guy? A staple to competitive battling since Gen I and I can sea why. Dragon Dance sets up Gyarados to be a strong sweeper. Earthquake, Waterfall, and Outrage are all picked for their power and coverage. Waterfall is picked over Aqua Tail for the better accuracy. Ice Fang is left out, but a good addition to this guy. Persim Beryy is there to help with the confusion after Outrage. Intimidate is always fun to run, and EVs and nature are to help sweeping.
Staraptor @ Leftovers
Ability: Intimidate
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Brave Bird
- Close Combat
- Quick Attack
- Pursuit
I feel that this bird is always a underrated, but it can make anyone bird-or-prey for their lives. Brave Bird is great for it's power combined with Staraptor's grat attack stat. Close Combat helps with anyone who would normally wall Staraptor, like Steel and Rock types. Quick attack for STAB and speed priority, and Pursuit for when they run from Staraptor. Leftovers help restore HP from Brave Bird's recoil, and Nature and EVs help with sweeping. And as previously stated, Intimidate is just fun.
Altaria @ Altarianite
Ability: Natural Cure
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Earthquake
- Return
- Heal Bell
- Dragon Dance
Another Dragon Dancer. Dragon Dance set's up Earthquake and Return, and once it Mega Evolves, Return gets Pixelated and turns into a Fairy type move, making the combo great coverage. It can really Fluff you up. Heal Bell is to take advantage of M-Altaria's bulk and add some support to the team. EVs and Nature are to add speed and power.
Klefki @ Leftovers
Ability: Prankster
EVs: 252 HP / 252 Def / 4 SpA
Bold Nature
- Thunder Wave
- Spikes
- Magic Coat
- Flash Cannon
The Key to this team. Thunder wave and Spikes are to slow down and hurt the opponent. Magic Coat is bounce anything odd back like stealth rock, will-o-wisp, or a stray toxic. Flash Cannon is to add a little offense and to avoid being taunt bait and to hurt any fellow fairy types. Leftovers is for recovery, and Evs and Nature are to add some bulk.