godan verb (-tsu), intransitive verb
Emphatic, colloquial form of 立つ, often implying a sudden or forceful standing up. Used in casual speech.
See also: 立つ
彼は急におっ立って部屋を出て行った。
He suddenly stood up and left the room.
髪の毛がおっ立ってるよ。
Your hair is sticking up.