Yes, everyone knows that your diet plays the biggest role in being able to see your abs, but you still have to train them. In fact, you have to build them. And that means using added resistance. Try these exercises.
This is one of the most underestimated core exercises in existence. It'll strengthen the obliques, erectors, multifidi, and even your glutes. You can use a rope, a chain, or a long bar to perform this movement. Toy around with different body positions and vectors. All work very well. Make sure you repeat with the opposite side.
Bodybuilders have used the kneeling cable crunch for years for good reason – it's a great dynamic abdominal exercise. You can experiment to figure out what set-up feels best, but make sure you're getting in spinal flexion (mostly in the thoracic region and not the lumbar region) and not just hip flexion.
The kneeling cable side-crunch is a very effective movement that will target the obliques better than the kneeling cable crunch.