First thing is first: stick to your basic movements that involves core involvement. Movements like squats, overhead presses, pullups, front squats, and maybe some direct core/oblique work.
I would recommend suitcase deadlifts over any other exercise for development of specific obliques.
For extra accessory oblique work, you could side planks:
Another method is when performing specific hanging leg raises, cross your feet for an equal amount of reps. Perform 10 reps of left leg crossed, then perform 10 reps of the right leg crossed.