While you can use the mouse to select any topic, it is also possible to navigate the outline entirely with the keyboard.
To navigate flat up and flat down:
Press ⌘-Up Arrow or ⌘-Down Arrow.
This selects the topic shown physically above or below the current topic, without regard to the hierarchy of the outline.
To navigate up and down:
Press Option-⌘-Up Arrow or Option-⌘-Down Arrow.
Navigating up selects the current topic’s previous sister, or (if the current topic is the first sister) the current topic’s mother.
Navigating down selects the current topic’s next sister, or (if the current topic is the last sister) the current topic’s next aunt.
To navigate up and down the hierarchy:
Press ⌘-Left Arrow or ⌘-Right Arrow.
Navigating up the hierarchy selects the current topic’s mother.
Navigating down the hierarchy selects the current topic’s first daughter.
Adding the Shift key to any of these keyboard shortcut causes Opal to perform multiple selection of topics as you navigate.