Transcription of the key portion of the Ash-Oak call:
Oak: Which pokemon are you going to use in the tournament? All the pokemon you've left with me are in top shape. I can send them at any time.
Ash: That won't be necessary.
Ash: When I came to Houen, I brought only Pikachu with me, because I wanted to try a fresh start.
Oak: Oh yeah, that's right...
Ash: Then, working hard with the pokemon I caught here, we got all eight badges. I'd love to let Charizard and the others fight now, after so long, but Grovyle and the rest of my team are ready and willing to battle. So for this tournament, I want to fight with just Grovyle and the rest of my current team.
Oak: I see. That's good determination. It's true that you'd have an advantage by battling using pokemon that are rare in Houen. But you'd rather have even battles without resorting to that sort of strategy, right?
Oak: Then I won't say anything more. Just use all the power you have, and battle to the best of your ability.