(setq org-startup-folded t) makes Org files open with all headlines folded, my attention-deficient preference.
But then diffs are hidden when comparing Org files with ediff! Here is my ediff config:
(use-package ediff
:custom
(ediff-split-window-function 'split-window-horizontally)
(ediff-window-setup-function 'ediff-setup-windows-plain)
:hook
;;; Start Org files in maximally-unfolded view
(ediff-prepare-buffer . #'outline-show-all))