Okk is a beefy goblin that can't attack if it's the biggest creature attacking.
Okk can't attack unless a creature with greater power also attacks.
As the Active player, during the Declare attackers step (508.1), I choose to attack with Okk, a random 1/1 goblin, and Goblin Piledriver. I think that Goblin Piledriver provides me an out, because it is boosted for each other attacking goblin, in this case making it stronger than Okk.
Whenever Goblin Piledriver attacks, it gets +2/+0 until end of turn for each other attacking Goblin.
If I declare Goblin Piledriver, Okk, and another Goblin attacking, does that satisfy the restriction portion of 508.1c?