Steve, never fooled with oaks as bonsai, but most of them need really good drainage, maybe a bigger screen for the soil mix or more sand? I have an old John Naka bonsai book somewhere, seems like he grew several native California oaks along with his California junipers. I'll see if I can look it up. What soil mix do you usually use? I think the last mix I settled on when I was growing bonsai was about equal parts screened sand/chat, Turface, and Nature's Helper. It worked pretty good for most things.