also: おう
おお、そうだったのか。
Oh, so that's how it was!
おお、なんてこった。
Oh no, what a mess.
おお、そういうことか。
Ah, so that's what it means.
Okay, leave it to me.
あっ is a short, sharp exclamation for sudden noticing or alarm, while おお is longer and more drawn-out, fitting wonder or hearty agreement.
Standard hiragana spelling for this interjection.
Variant spelling; pronunciation is the same as おお.
Emphatic or drawn-out spelling using long vowel mark.
Katakana spelling, often used for emphasis or in manga.
A natural exclamation; the exact origin is uncertain. The kanji 応 is an ateji used for its sound, not its meaning.