D'oh! I forgot about Choice Scarf Flygon. Also, Ono can 2HKO Scizor with EQ after a DD with a reasonable amount of attack investment, so if Ono carries Leftovers, it can take two BPs (or not, if Scizor gets really lucky) and survive at ~1%-~16% health (at least with the spread I suggested above, I don't know how other spreads take it). Without Leftovers, it has ~25% chance to be KOed by two BPs (according to my calcs).
The reason I used 252 Atk Adamant was because DD Ono does a number to offensive teams without priority and/or a Scarfer (and as you mentioned, Haban helps with Scarfers), due to its great speed after a DD and amazing attack, so I wanted it to deal a bit more damage to Hippowdon and the like, which would otherwise wall it.
But now that you've mentioned Flygon and Garchomp, I think Haban > Leftovers, because that's what many teams will be using to check it (I think) and it allows Ono to get past them and SWEEP.
btw, I think that DD Ono will probably work best on a hyper offensive team, with its nice resistances and good speed allowing it to switch in after a kill and setup, while its iffy defenses prohibit switching in on very many moves. Just a random thought.
Finally, I've been wondering what to put as the fourth move on a DD set. Along with DD, Dragon Claw, Outrage, and EQ, I've been wrestling with Taunt, Dragon Tail, and Brick Break. BB deals good damage to Skarm (Ono's biggest counter), but it allows Skarm to WW it away. On an all-out 252/252/4 set, Skarm's Brave Bird does ~50% to Ono, so using Brick Break might be the best choice, dealing ~60% to Skarm over two moves before Skarm KOs. However, Taunt allows it to setup on things like Cress while stopping Roar/WW. Dragon Tail also stops Roar/WW and allows Ono to get some extra damage on the opponent's team, but doesn't stop TWave and stuff. I think Taunt is probably best for the SD set, as it's meant to break through walls. But would Taunt or BB work best for DD? BB allows it to get that extra damage on Skarm, but for a set with more defensive EVs, it could just run Taunt and setup more on Skarm. I'm leaning toward Taunt, but I'd like some input.