OutlineMagic

Extends OutlineMode and OutlineMinorMode with the command outline-cycle – a single command to toggle hiding/showing.

Available at https://github.com/tj64/outline-magic

After installing it into a location on your load-path, enable it with:

(eval-after-load 'outline
  '(progn
    (require 'outline-magic)
    (define-key outline-minor-mode-map (kbd "<C-tab>") 'outline-cycle)))