The interjection 哦
An interjection (感叹词) is a word or expression that occurs as an utterance on its own and expresses a spontaneous feeling or reaction. It expresses feelings and emotions but does not name them. It is usually placed at the beginning of a sentence.
The interjection 哦 has two functions, which can be distinguished based on pronunciation:
1) 哦 
indicates doubt or surprise
In English it can be translated as "really?", "oh!".
indicates doubt or surprise
In English it can be translated as "really?", "oh!".Examples
哦，真的吗？
Oh? Is it so?!
ó, zhēn de ma
哦！会有这种的人？
Oh, these types of people also exist?
ó! huì yǒu zhèzhong de rén
哦，原来是这样！
What! How can that be!
ó, yuánlái shì zhèyàng
2) 哦 is used when a speaker either realized something or guessed about something
In English it can be translated as "ah!", "oh!“.
is used when a speaker either realized something or guessed about something
In English it can be translated as "ah!", "oh!“.Examples
哦，我明白了！
Oh, I see!
ò, wǒ míngbái le
哦，我觉得他为你准备一个惊喜！
Ah, I guess he is preparing a gift for you!
ò, wǒ juéde tā wèi nǐ zhǔnbèi yígè jīngxǐ