The 'oi sound' /ɔɪ/ is a two-sound vowel that ends in a brief 'y sound' /y/.
Part 1: At the beginning of the sound the jaw is open about mid-way and the body of the tongue is in a low position in the mouth. The bottom teeth can be felt along the sides of the tongue.
Part 2: Then the jaw closes slightly. The body of the tongue moves upward until it is near the tooth ridge, similar to the position of a 'y sound' /y/. The front sides of the tongue touch the inside of the top, side teeth.