I want to create a minor mode that helps to input thai tone character. This is what I wrote:

(defvar thai-input-mode-map
  (let ((map (make-sparse-keymap)))
    (define-key map "2" (lambda () (interactive)
              (replace-string "o" "ô")
              ))
    map)
  "Keymap used by `thai-input-mode'.")
(easy-mmode-define-minor-mode thai-input-mode
                              "Input mode for Thai"
                              nil  " TH" thai-input-mode-map)
(global-set-key "\C-x\C-t" 'thai-input-mode)

Of course I must implement more details, but I'm already have problems. First of all the replace-string function doesn't work even if there is o in a buffer. Secondly I couldn't figure out how to replace only a character that is at the cursor.

How can I write to implement that kind of function?

I want to create a minor mode that helps to input thai tone character. This is what I wrote:

(defvar thai-input-mode-map
  (let ((map (make-sparse-keymap)))
    (define-key map "2" (lambda () (interactive)
              (replace-string "o" "ô")
              ))
    map)
  "Keymap used by `thai-input-mode'.")
(easy-mmode-define-minor-mode thai-input-mode
                              "Input mode for Thai"
                              nil  " TH" thai-input-mode-map)
(global-set-key "\C-x\C-t" 'thai-input-mode)

Of course I must implement more details, but I'm already have problems. First of all the replace-string function doesn't work even if there is o in a buffer. Secondly I couldn't figure out how to replace only a character that is at the cursor.

How can I write to implement that kind of function?

Edit

By following Gilles's advice I wrote like this:

(defvar thai-tone-conversion-list
    '(("o" . "ô") ("u" . "û")))
(defun thai-add-tone ()
  (interactive "*")
  (when (let ((case-fold-search t))
          (looking-back "[ou]"))
    (replace-match (assoc (match-string 0) (thai-tone-conversion-list)))))
(defvar thai-input-mode-map
  (let ((map (make-sparse-keymap)))
    (define-key map "2" 'thai-add-tone)
    map)
  "Keymap used by `thai-input-mode'.")
(easy-mmode-define-minor-mode thai-input-mode
                              "Input mode for Thai"
                              nil  " TH" thai-input-mode-map)
(global-set-key "\C-x\C-t" 'thai-input-mode)

But I get assoc: Symbol's function definition is void: thai-tone-conversion-list error, when I put 2 in thai-input-mode.