The interjection 呦
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 呦 
sound can be made either when expressing surprise, or at the end of a sentence as an exclamation. It can be translated as "oh!".
sound can be made either when expressing surprise, or at the end of a sentence as an exclamation. It can be translated as "oh!".Examples
呦，电怎么停了呢？
Oh! Why did the electricity go out?
yō, diàn zěnme tíng le ne
呦，你掉护照了！
Oh! You dropped your passport!
yō, nǐ diào hùzhào le
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