What is the meaning of онъ?

he (personal pronoun)

that one there, that one yonder

Pre-1918 spelling of он (on).

Letter г (g) in the genitive/accusative case ending is pronounced as /v/.

The alternative forms starting with н- (n-) are used after a preposition.

Archaic forms: ея́ (jejá), нея́ (nejá).

Instrumental forms ending in (-ju) are either dated, poetic, or dialectal.

The prepositional case is never used without a preposition.

Source: wiktionary.org