how many more times do we see oz in the show after he left?

Actually, we see Oz only in another ep, which is "New Moon Rising" (Btvs 4x19), when he comes back to Sunnydale just to know about the rising relationship between Willow and Tara and be caught from the Initiative.
By the way, the last time we see him is in the season finale "Restless" (Btvs 4x22) in Willow's dream.