As you start working out on your belly, your external oblique automatically gets worked on and remember oblique is the outermost muscle of the abdomen.
Some of the exercises for the abdomen are as follows:
1. Bicycle movement –Lie down on the ground flat onyour back. With your hands up beside your head, lift your legs off the ground to a 45 degree angle. Exhale and contract the abs to lift your torso off the ground to a 45 degree angle as well. Contracting the abs, move the legs in a pedaling motion, touching the left elbow to the right knee. Alternate and repeat. Focus on keeping the rectus abdominis contracted during the entire exercise.
2. Another exercise that you can do for your external and internal oblique is to lie on your back, with your left hand supporting your head, elbow pointing forward. Lift both legs off the floor, taking your right leg over your left, crossing at your knees. Inhale and slowly contract your lower abs, bringing your right knee up towards your chest, while lifting your upper body, focusing on taking your left elbow to your right knee. Exhale and slowly go back to the starting position, knees turned out (feet off the floor), shoulders lowered back to the floor. Work one side at a time, and then change sides both arms and legs.
3. Plank pose- Lie on the floor with your face down, feet together. Prop your torso up on your elbows that are positioned directly underneath your shoulders. Clasp your hands together, tuck your toes under (the same way you would if you were about to do a push-up. Locking your knees, extend your hips (imagine trying to drive your toes into the ground). You should now be hovering over the ground with your body weight spread between your forearms and your toes. Your body should now be in a straight line horizontal to the ground. Hold on to this plant position for as long as you can, may be a count of forty would be perfect.