The question I have is that I have a Goblin Piledriver, a Raging Goblin, a Goblin Raider, and Okk in play. If I declare the Raging Goblin, the Goblin Raider, and the Piledriver as attackers, will I then be able to attack with Okk also due to the Piledriver's increased power?
A: The answer is no. The first thing you do in the Declare Attackers Step is announce with which creatures you're attacking (rule 308.1). You announce Okk, Piledriver, and Raging Goblin.
The second thing you do (rule 308.2) is decide if the attack is legal. It's not, because the Piledriver's triggered ability hasn't yet resolved. It won't until your attack has been declared legal and then you get priority (rule 308.6).