9 different plank variations to train the obliques. Many of these plank exercises are also great for developing core stability.
Planks are often underrated, but in reality they work the rectus abdominis and transverse abdominis muscles very effectively. With little changes to the conventional plank, you can also emphasize the obliques.
Obliques is one of the most functional muscle groups of the human body because they are behind the rotational power that is needed in martial arts and sports.
If you want powerful kicks or devastating punches, you need to train the obliques. The more mobile and stronger your obliques become, the more torque you have for producing power.
Your abdominals will also never look "complete" and aesthetic without the presence of at least some level of obliques development. Moreover, your abdominal growth can be restricted by the lack of growth in your obliques because the body strives for structural balance.
Despite the importance of obliques, the exercises and training of obliques is shallow and simple in the fitness industry, among martial artists and even among many athletes. It's normal to train the obliques with just 1 exercise after your workout, but this is far from enough.
Obliques is a massive muscle group that covers lots of area in your body. There are also 2 different oblique muscles and they have 3 different articulations of the human body. It's impossible to cover all of these functions with just 1 or 2 exercises.
The reality is that you can spend one hour training your obliques because there is so much to train and so many different exercises need to be utilized to train the obliques to their full potential.
Abs 20XX guide covers everything you need to know about training the obliques. In addition all parts of the abs are also covered and there are 10 core routines to perfect the core. Abs 20XX is a 95 page e-book that works on any device that can open PDF files.

One Abs 20XX page about training the obliques.

Page from Abs 20XX.
In addition one section of the book is just diet & nutrition so you can lose the excess fat over your abdominals. In the end there is a comprehensive Q&A section regarding training and nutrition.
You can find more info and Abs 20XX from this link.
Train hard, stay safe.