Classes are used to apply the same styles to multiple elements. All styles set for the class will be applied to all elements that are assigned that class.
To add a class to an element, click Choose selector... or the ID of the currently active element, and then type the class name and click Add Class. Oxygen will automatically suggest classes that match your input.
To switch between editing styles for the ID and class, simply click ID or Class to open the class dropdown.
To remove a class from an element, click the X button next to the class name.
A list of all classes present on your site can be found at Manage -> Selectors.