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.
In Mainland Simplified Chinese.
This is the most frequently used word.You can say it exactly the pronunciation of the word. and other three words can be used in writing.It's weird in speaking.
哦：Oh well，I see
（Some people will think you don't have interest in what he just said if you response a "哦" with nothing，incoming 哦 will piss him off soon)。
喔：1:Oh,2: a cock's crow(literally chicken)
Often use it to express accent. e.g. 好，accent edition：好喔。
You can see it more in old novels。can be replaced by 哎.