Cleon
Legend
OK, starting with the basics...
I don't have stats for a red oak tree, but the death's head tree was detailed in Dragon #339...
Well, I remember seeing stats for trees in the Artesia roleplaying game, but that's not D&D. (And yes, they were ordinary, mundane trees).
Death's Head Tree
Advancement: 11-20 HD (Huge); 21-30 HD (Colossal)
This suggests Colossal size to me, or at the very least Gargantuan.
So we've got 30 HD, AC 3, high spell resistance, and immunity to fire.
Well that Death Head's Advancement conveniently maxes out at 30 HD, the same as the charmer we're converting, so I'd like to make it Colossal.
Would you post the Flying Heads SA too? The tree of wailing souls appears to have a more powerful version of it.
This suggests high mental ability scores to power these spell-like and psi-like abilities and potential spellcasting/manifesting.
Since it doesn't demonstrate much tactical nous I'm thinking we should keep the Intelligence low but give it an impressive Wisdom and exceptional Charisma.