What is the meaning of On-?

on, at, toward, upon.

un-, in-

bad, grave, horrifying

on, without

Rōmaji transcription of おん

un-, in-

un-, in-

forming verbs and adjectives from participles and nouns, with various senses of on, upon, to, there, thither

in, into; prefix used to intensify an action or to denote origin or initiation

denoting reverse action

off, away

allomorph of ön- (negative/sociative irrealis prefix) used for stems that have a first vowel o or u.

Source: wiktionary.org