One of the most time-consuming operations in Oxygen Builder is adding classes, or even just testing other classes to see if they are correct. Class picker adds a dropdown toggle in the left-hand styles panel that reveals an alphabetized list of your classes. Classes that have already been applied to the selected component are displayed in green. Click a white class name to immediately add the class to the component. Click on a green class name to immediately remove it.
This functionality is perfect when integrating with CSS frameworks for Oxygen, as you can toggle classes on and off quickly, rather than having to memorize and type them every time.