In WeChat, QQ, text messages, and the like, it is common to see 哦, 噢, and 喔 used to express an acknoledgement that the other person has said something. They all seem fairly close equivalents to the English "Oh" to me. In my personal experience in Mainland China, I would say the frequency of use is in the order that I listed them.
I'd like to know, in common usage, are they interchangeable? Are there subtle differences between them? In a more strict grammatical sense, is one more correct than the others?
Note: I am not asking how these characters are used at the ends of sentences. I am asking about their meanings when used alone. This question is different than the previous question 啊，诶，喔，哦：compare and contrast.