The oblique muscles get their name because they are oriented on an angle. They attach various points of the ribs to the pelvis.
Interestingly, the internal oblique and external oblique actually work quite well when you're doing a basic crunch moving straight forwa
However, the obliques work at maximum effectiveness when you do twisting crunches or rotational motions. In addition, the obliques work at maximum effectiveness when your body does side bending movements.
So if you want to work your obliques as much as possible, you need to incorporate rotational exercises, twist crunches, and side bending motions.