哦 (ó/ò) is an interjection expressing surprise, realization, or acknowledgment. Its tone varies with meaning: a rising tone suggests surprise, while a falling tone signals understanding or mild agreement.
哦,原来是这样,我终于明白了。
ó, yuánlái shì zhèyàng, wǒ zhōngyú míngbai le.
Oh, so that's how it is — I finally understand.
哦?他居然辞职了?
ó? tā jūrán cízhí le?
Oh? He actually resigned?
哦,好的,我知道了。
ó, hǎo de, wǒ zhīdào le.
Oh, okay, I see.