More on the -おう form.

Turns out that, whatever it is, the same thing that's happening with 有難い・ありがとう is happening with おめでとう (愛でたい/目出度い) and おはよう (早い). So why don't we say おありがとう?

And now we have めでたい. Per Kotoba, the kanji for めでたい　is 愛! お愛でとうございます。 Kotoba also lists two ateji forms of めでたい, 目出度い　and 芽出度い.

Well, one more: オフレコ, a mash of off the record.　僕は日本語が上手じゃないけど、オフレコだよな。