哦 is an interjection，and only can be used seperately. 哦（4）is used to show one's acceptance to other's idea. eg.哦，我明白了（oh，I understand） 哦（2）is used to show one's doubt or exclamation. eg.哦？他们在说我？（oh？they are talking of me?）

用这个有时候会给人你呆呆的感觉的，哈哈哈哈

哦has three pronunciation 1o:second tune,it's a interjection,to show surprise or asking:哦，是这样的吗？哦，是那么一回事。 2o:four tune ,it's also a interjection ,to show you have understood:哦，我明白了。 3e:second tune,interjection as well,to discribe sing softly and lowly

Hi~~~Generally speaking,you can use 哦 when you understand what the other people have said to you.For example,if I tell you that it will rain tomorrow morning,you can say "哦，我知道了,明天出门我会带伞。"It means "Oh,I got it.I will take an umbrella when I'm out." And another example:Your mother tells you that it's time to go to bed.You say:"哦，好。”It means you have understand other people's request and you did not reject it.