The Weighted Side Bend really isolates the obliques, and is super simple to do. All you need is a dumbbell or plate weight.
Stand with your feet shoulder width apart and the weight in one hand. Bending sideways at the waist, lower that weight to around knee level and straighten back up. After finishing your desired reps, change the weight to your other hand and repeat on the opposite side. Remember to bend only at your waist. If you experience lower back pain, lower your weight.
Weighted Side Bends look like this.