Next Sunday we're planning to throw a party for Mr. Oka.
Literal
Next-week [possessive-の] Sunday we [topic-は] Oka-teacher [for-のための] [attributive-の] party [object-を] do [intention-つもり] is.
~のため here is the 'for the sake of / in honor of' sense (different from the causal 'because of' ). のため + の + [noun] turns it into an attributive modifier. 先生 is an honorific for teachers, doctors, and respected authorities.