The method that comes to mind is a bit complex: You create a straight-line version of that “brush stroke” and turn it into an art brush, then apply it to the stroke of a circle. That’s a really basic description, but here are more details about defining custom brushes:
https://helpx.adobe.com/illustrator/how-to/illustrator-create-art-brush.html
Kalleheikki Kannisto
Senior Graphic Designer