I agree that for the most part he is outclassed as a Set-up Sweeper by Mence and Chomp, mainly due to the unfortunate speed, but let's remember that these are the two Pokemon that broke 4th Gen OU. I wouldn't underestimate this beast.
He has several unique advantages. Dragon Tail could be great on with a CB, and also, as mentioned above lets it beat Skarmory who try to Whirlwind it. Taunt fulfills the same use, and also lets it decimate stall teams. In fact, I would argue that Ononokusu is much more effective against stall than most other dragons are, thanks to Taunt, Ridiculous Attack, Swords Dance, (which most Dragons lack), and no exploitable 4x weakness. It's still great against faster paced teams with DD and Scarf. I think this set could be monstrous if Scarfed dragons (read:Garchomp) become popular revengers.
Ononokusu @ Haban Berry
Adamant: 126 Hp/252 Atk/126 Spd (tweak as needed)
-Dragon Dance
-Outrage
-Earthquake
-Taunt/Dragon Tail/Dragon Claw/Swords Dance
DD as they switch in their Scarf Dragon, DD again as you survive with Haban Berry, then start rampaging as you kill the dragon and obliterate the rest of the team with +2 Atk/Spd. If they send out skarmory, use Taunt or Dragon Tail to continue your Set-up.