Seated Oblique Twist Stretch
There are many good oblique stretches, but I included this one, because it can be done almost anywhere, is easy to change the intensity of the stretch, and it can stretch some of your back as well.
Start by sitting in a chair or on the floor. The stretch is a little different in between the chair and floor, so use whatever feels best to you. In either case, you will start by twisting away from the side you want to stretch. For example, if you want to stretch your right side, start by rotating your torso to the left.
If your obliques are really tight, this could be enough to give you a decent stretch, but most people will probably benefit from a little more intense stretch. To increase the stretch, use the hand/arm of the side you are stretching and place it over the opposite leg. In this case, you would put your right arm over your left leg.
If you are on the floor, you should have your leg bent, so your knee is elevated, and you can place your arm across your thigh. Your elbow will probably end up somewhere around your opposite knee, but adjust this position as needed.
When sitting in a chair, your arm will still go across the opposite leg, but you might have your wrist across the opposite thigh or you may need to hold your leg with your hand. In either case, the goal is to use your arm/hand to apply pressure against your leg. This provides extra leverage so you can twist your body further and get a more intense stretch.
If this is still not enough of a stretch, you can use your other arm (the one away from the side you are stretching), to further increase the stretch. When sitting on a chair, you can use the other arm to hold on to the chair to provide extra leverage to twist. When stretching on the floor, the you can reach further behind you with the hand and push against the ground to provide the extra leverage.