It can be heart-stopping to realize you've just modified a class when you meant to make changes to an ID. Class Locker adds a handy toggle operation to prevent you from making changes to classes.
By activating this extension, you'll find a small lock symbol to the right of the selector dropdown in the Styles panel. When the icon is blue, classes are locked and you will not be able to edit them. The operation uses a timer to check whether you've selected a class, and will quickly change your selection back to ID if it detects class selection. This means you may see a small flicker, which does not indicate a problem.
An additional option is available to start Oxygen with classes locked. First, enable the Class Locker extension and save your settings in WP Admin. You will now see the option, which you can select and save your settings again.