啊，诶，喔，哦：compare and contrast The characters in the title are, FWIK, all exclamatory sentence-final particles, though I'm not too sure about ei. So I was wondering what the difference was between the three. Is there a difference in the first place? And if so, what is it?
哦 (Ò), I see!

哦 (O), is that really so?

喔 (Ō), I see!

诶 (唉, Āi), good heavens!

啊 (Á), you are kidding me, right?

啊 (Ǎ), is that really so?

啊 (À), OK.