(おお)

おおく
noun, adverb
many; much; a large number; mostly
quantity
1. many; much; a large number; the majority
A large number or amount of something. Functions as a noun, often followed by の (多くの = many) or used with が/は as a subject meaning 'most' or 'the majority.'
Many people participated.
Most of the students use the train to commute to school.
In this area, it's inconvenient without a car in most cases.
被害(ひがい)()けた建物(たてもの)(おお)(ちく)30年以上(ねんいじょう)ものだった。
Most of the damaged buildings were over 30 years old.
2. mostly; for the most part; in many cases
Used adverbially to indicate that something is true in the majority of cases.
In this area, it's inconvenient without a car in most cases.
Japanese people for the most part don't talk a lot.
I spend most of my days off at home.

The noun/adverb form of (おお) (many). While (おお) exists as an adjective, it cannot directly modify a noun — *(おお)(ひと) is ungrammatical. Use (おお) instead.

Common patterns

Watch out

Learners often write (おお)(ひと) instead of (おお)(ひと) or (ひと)(おお). This is one of the most common errors with (おお).