Otis Up is a unique compound exercise that works the upper abdominal muscles but also includes a pressing action to target the shoulders.
With the pressing part of the exercise, your core has to remain tight in order to stabilize the upper body and support the lower back.
It is better to use a lighter weight and a high number of reps for this exercise. Using a heavy weight might cause you to break your form, and hurt your lower back. focus on technique rather than on weight.