This is the 5503rd most frequent Japanese word.
おおっ
An exclamation expressing surprise, amazement, or excitement, similar to "Oh!" or "Wow!" in English.
Here, 'おおっ' is used to express surprise and admiration towards the discovery.
おおっ、これはすごい発見だ！
Oh, this is an amazing discovery!
In this sentence, 'おおっ' conveys the speaker's recognition and surprise upon noticing the arrival.
おおっ、君が来たのか。
Oh, you came.
Here, 'おおっ' signals the speaker's alertness and caution towards the situation.