Deoxys-S

Late Game Sweeper
Item: Life Orb
Nature: Hasty
EV's: 160 Atk, 252 Sp.Atk, 98 Speed
Moveset:
Superpower
Ice Beam
Psychic
Shadow Ball/Thunderbolt (Depends on your team)

Strategy Deoxys-S is probably the best late game sweeper in OU. Superpower handles Blissey. Ice Beam brings down Dragons and various other types. Psychic gets STAB. Depending on your teams type coverage Shadow Ball or Thunderbolt will be good.

Dragon9's Version Of Deoxys
Item: Life Orb
Nature: Hasty
EV's: 160 Atk, 252 Sp.Atk, 98 Speed
Moveset:
Calm Mind
Thunderbolt
Psychic
Superpower

Strategy Bring Deoxys-S in as if it was a revenge killer, well thats what your opponent will think. Your opponent has a Gyarados out, he thinks your gonna thunderbolt him, so he switches out. Instead of Thunderbolting your really gonna Calm Mind up. Now your at x1.5 sp.atk. Your very deadly now and can easily sweep a non-prepared team. Magnezone obviously walls T-Bolt and Psychic but then you have Superpower which will hopefully take Magnezone out. Enjoy !

Counters
Deoxys-S can be stopped by anything that walls sp.atk's except for Blissey since superpower wrecks Bliss. Of course since it's a late game sweeper, it may not easily be stopped and may 'cause havock for your team. You could always try stalling it out since it'll have Life Orb.