Your eval-after-load is close to correct. Check out C-h f org-indent-mode (after Org is loaded):

org-indent-mode is an interactive autoloaded compiled Lisp function in `org-indent.el'.

Diminish hacks directly into the variable minor-mode-alist, so the call to it must occur after org-indent.el has been loaded, not just org.el:

(with-eval-after-load 'org-indent
  (require 'diminish)
  (diminish 'org-indent-mode))

Your eval-after-load is close to correct. Check out C-h f org-indent-mode (after Org is loaded):

org-indent-mode is an interactive autoloaded compiled Lisp function in `org-indent.el'.

Diminish hacks directly into the variable minor-mode-alist, so the call to it must occur after org-indent.el has been loaded, not just org.el:

(with-eval-after-load 'org-indent
  (require 'diminish)
  (diminish 'org-indent-mode))

Your eval-after-load is close. Check out C-h f org-indent-mode (after Org is loaded):

org-indent-mode is an interactive autoloaded compiled Lisp function in `org-indent.el'.

Diminish hacks directly into the variable minor-mode-alist, so the call to it must occur after org-indent.el has been loaded, not just org.el:

(eval-after-load 'org-indent '(diminish 'org-indent-mode))

Your eval-after-load is close to correct. Check out C-h f org-indent-mode (after Org is loaded):

org-indent-mode is an interactive autoloaded compiled Lisp function in `org-indent.el'.

Diminish hacks directly into the variable minor-mode-alist, so the call to it must occur after org-indent.el has been loaded, not just org.el:

(with-eval-after-load 'org-indent
  (require 'diminish)
  (diminish 'org-indent-mode))