Here are some great stretches for the External Obliques!
Since the external obliques has contralateral (opposite) rotation, our first stretch is a same side rotation to elongate the muscle. While seated on the floor, stretch both legs out straight in front of you. Cross one foot over the knee of the opposite leg. Then hold the knee to your chest, or push off of it using your elbow, and rotate to the same side as the raised knee. Hold for 10-15 seconds then switch sides.
Another stretch to elongate the External Oblique is to do a lateral bend. While standing raise one arm up and over your head as you lean to the opposite side (of the raised arm) without lifting the hip. Hold for 10-15 seconds, then repeat on the other side.