interjection
Casual exclamation of mild surprise, realization, or a small mistake. Often used when noticing something or making a minor error.
おっ、もうこんな時間か。
Oh, it's already this late.
おっ、ごめん、ぶつかっちゃった。
Oops, sorry, I bumped into you.
A natural exclamation; the exact origin is uncertain.