One of the big faults in the Overhead Squat is that people follow an incorrect line of action. They neglect to send their butt back and down when initiating the descent. Therefore in order to fix this I recommend the Seated Overhead Good Morning. Start seated on a bench or low box with the bar in the overhead position with a snatch grip. A narrower grip will be more challenging and can be utilized for deeper stretching. Keep the knees out and the shins vertical to the floor. Lean forward at the hips, rolling to the front of the sits bones. Keep the shoulders active and keep pulling the bar back. Try to drop your chest between your knees while stabilizing the bar overhead. Return to an upright seated position. Do a lot of reps of this. You can also perform box squats with the bar overhead.