(defun org-outline-numbers (&optional remove-p)
  "Add outline number overlays to the current buffer.
When REMOVE-P is non-nil (interactively, with prefix), remove
them.  Overlays are not automatically updated when the outline
structure changes."
  ;; NOTE: This does not necessarily play nicely with org-indent-mode
  ;; or org-bullets, but it probably wouldn't be too hard to fix that.
  (interactive (list current-prefix-arg))
  (cl-labels ((heading-number ()
	       (or (when-let ((num (previous-sibling-number)))
		     (1+ num))
		   1))
              (previous-sibling-number ()
	       (save-excursion
		 (let ((pos (point)))
		   (org-backward-heading-same-level 1)
		   (when (/= pos (point))
		     (heading-number)))))
	      (number-list ()
	       (let ((ancestor-numbers (save-excursion
					 (cl-loop while (org-up-heading-safe)
						  collect (heading-number)))))
		 (nreverse (cons (heading-number) ancestor-numbers))))
              (add-overlay ()
	       (let* ((ov-length (org-current-level))
		      (ov (make-overlay (point) (+ (point) ov-length)))
		      (ov-string (concat (mapconcat #'number-to-string (number-list) ".")
						     ".")))
		 (overlay-put ov 'org-outline-numbers t)
		 (overlay-put ov 'display ov-string))))
    (remove-overlays nil nil 'org-outline-numbers t)
    (unless remove-p
      (org-with-wide-buffer
       (goto-char (point-min))
       (when (org-before-first-heading-p)
	 (outline-next-heading))
       (cl-loop do (add-overlay)
		while (outline-next-heading))))))