Open the sidebar, and click on the Styles tab. In the second section, you’ll see all the default Saved Styles (“Heading 1”, “Highlight:Yellow”, “Purple”, etc.) The first ten of them have hotkeys that will invoke them.
If you click on any of them, you can change its attributes in the Inspector. Then right-click on the Saved Style and change the name (if you like).
One caveat: I wasn’t able to get this to work with function-keys. So I went into OO Preferences/Keyboard and changed “Apply Named Styles with” from Function Keys to Control+number.