Kahuna not in OED? Oh that's right, they killed Captain Cook before he could get back with the definition.

Generally translates to (from many polynesian tongues/dialects):

A leader, wise man, shaman, medicine man, conjurer.

Something akin to Merlin?

Note: it now means something quite different. The Outrigger Waikiki has a "Big Kahuna" party every week with VERY loud music and free-flowing mai tais...