The most important thing to remember here is that you need a low enough bodyfat percentage in order to see the oblique muscles. The same applies to your abs. So it's important that your diet is correct in order to reach a bodyfat percentage that is low enough to begin to see the obliques. To lower your bodyfat percentage, you need to widen the gap between calories burned vs. calories consumed. The "calories consumed" part of the equation should obviously be lower than the "calories burned" part. The bigger the caloric deficit, the more fat you will lose and the more your bodyfat percentage will drop.
From a workout perspective, performing a combination of ab crunches, twisting ab crunches, and oblique specific exercises is ideal. A great exercise to target the obliques is performed by holding a dumbbell in one hand down to your side. Keep your lower body stationary as you drop your torso directly to whichever side you're holding the dumbbell, and then raise back up. This targets the obliques and will help you put on oblique muscle.
It's very important to understand that you need a low bodyfat to see your obliques. Otherwise if your bodyfat is too high, it doesn't matter how many oblique exercises you perform, you will simply never see them. In fact, you will make it worse because the added muscle will only add size underneath the fat, which can give you the appearance of having a higher bodyfat percentage.