Either of these worked for me:

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

(add-hook 'org-indent-mode-hook (lambda () (diminish 'org-indent-mode)))

Either of these worked for me:

; whacka's answer
(eval-after-load 'org-indent '(diminish 'org-indent-mode)) 

; fixed Gastove's answer
(add-hook 'org-indent-mode-hook (lambda () (diminish 'org-indent-mode)))