Humans have two internal oblique muscles, which are located on either side of the rectus abdominis (the "six-pack" muscles). One quick way to locate them is to place a hand below the armpit and follow the side of the torso below the ribs.
The oblique muscles assist in rotating and bending the torso. The left oblique muscle contracts when bending the torso to the left, and the right oblique muscle contracts when bending the torso to the right. Their location below the ribcage also allows them to help in supporting the abdominal wall and in controlling internal pressure during respiration, especially during exhalation.