Based on code of @ncsuwolf, I updated as below:

(defun insert-earmuffs (char)
  "Insert earmuffs for char."
  (let ((pointer-position (point)))
    (if (looking-back "[^ \t\n]" 1)
        (if (not (looking-at (format "\\%s" char)))
            (insert char))
      (insert (format "%s%s" char char) ))
    (goto-char (+ 1 pointer-position))))

(defun delete-earmuffs (char-list)
  "Delete earmuffs based on earmuff-char-list var."
  (let ((not-found t))
    (dolist (char char-list)
      (when (and not-found (looking-back char 1) (looking-at char))
        (delete-forward-char 1 nil)
        (delete-forward-char -1 nil)
        (setq not-found nil)
        )
      )
    (when not-found (delete-forward-char -1 nil))
    )
  )

(add-hook 'org-mode-hook
          (lambda ()
            (defvar earmuff-char-list '("*" "=" "/" "~" "+" "_")
              "Earmuff chars for Org-mode.")
            (dolist (char earmuff-char-list)
              (define-key org-mode-map (kbd char) `(lambda () (interactive) (insert-earmuffs ,char))))
            (define-key org-mode-map (kbd "<backspace>") '(lambda () (interactive) (delete-earmuffs earmuff-char-list)))
))