喔、哦、噢 can also be used for that, but there are differences: 喔: The dictionaries say it signifies surprise, sudden realization or anguish. Personally I see it more often used at the end of sentences as a modal word (an effeminate way of ending a sentence) 哦: when pronounced 2nd tone, expresses surprise and disbelief; when pronounced 4th tone, means "Oh, I see now." 噢: When I suddenly realize I did something wrong or I've forgot to do something I'll use this word but not the other three, because it has some sense of regret and disappointment. It's also use for surprise in general. In fact the choice of these words may just be a personal habit. Each of my friend stick to their own favorite one when sending SMS, and I understand them even I myself would use another... So no need to really distinguish them seriously

喔喔can imitate the sound of cock crow. 喔,哦,噢when used alone doesn't make any sense. The meanings of 喔,哦,噢 rely on the rest part of the sentences . 哦is more frequently used than the other two. Anyway you can pick any of them as you wish as they are really no difference for many of our native speakers, and the usage totally depends on personal favours. I'd like to recommend 哦. It's easy to recognize and remember.

for me there's no difference. useless characters, just for making a sound.

木叶丸 je，he's right.

喔噢，Upstairs really explain in detail