Skarmory is best used as a Spikes Lead and is mainly used as a check for tough physical foes. In a tier like OU, you seldom see dedicated Special Attackers running around and murdering teams, so the current condition of OU is mainly why Skarmory's niche is so useful.
Skarmory works best as a Stall or Scout of sorts, which is a rather strange set of possibilities. As a Stall, Skarmory mainly uses Toxic and Protect alongside Roost and STAB to cut down tough opponents slowly but surely. This path is usually more unorthodox, as it's just...Plain.
As a Scout, Skarmory's used as a counter to switch into tough attacks. Thanks to Base 140 Defense and a slightly average Base 65 HP, it's a tough cookie, and can sponge hits fairly well, provided they're not special-based or super-effective. It usually switches in on Dragons, setup sweepers, and Ground-types (the standard Garchomp has a hell of a time trying to do away with it, actually), and sets up Spikes after the free turn. Then Scout Skarmory either attack with STAB moves, use U-Turn to switch in a more suitable team member, or even use Roar to phase and abuse the hazards you may've set up.
Skarmory isn't AS good as it was in the last few generations, as threats like Volcarona, Eelectross, and Hydreigon have risen as special attackers, but Skarmory is still regarded as the "Physical Blissey", as its good typing and amazing physical bulk make it great for checking all of those burly, tough physical attackers. With Swords Dance and Base 80 Attack, you could even sweep with Rock Slide, STAB, and X-Scissor.
He's got variety, stats, typing, and utilities; it's no surprise for him to be in OU, although he's a bit more lower-end now, especially since Roost is only open to him in the Dream World.