Your obliques are part of the abdominal muscular, and they're the muscles that help to rotate your spine. So, any type of abdominal exercise where you are doing rotation is going to fire up the obliques. A common oblique exercise is like a rush & twist. You can do with the body weight like how I am going to show you now or you could hold a medicine ball and make it a little bit more difficult. But the key things you want to think about are first engaging the deep layers of abdominals. So you're gonna just think about like you're pulling your belly button in, as if you're trying to touch your belly button to your backside, and then pulling in and lowing yourself down. The lower you go, the harder it's going to be to maintain that core contraction because the higher you are the less gravity is kind of coming down on you. The lower you are, the more force that you have to work against. So pulling the belly in and maintaining that concave position in your belly so you are firing up your superficial layer as well. You just start to rotate from one side and it's that rotation that's going to get in to the obliques. If you want to make it harder, you could add a weight or pick up speed. You could lift your legs up, make it a little less stable. Another oblilque exercise would be the bicycle crunch. So again to engage the obliques any exercise where you are doing rotation is going to be fantastic.