Where does “ono” come from?

ono (Slovak) comes from Proto-Slavic ono, from Proto-Indo-European h₂en — there.

ono (Slovak): it

Definitions

Step	Language	Word	Meaning
1	Proto-Slavic	ono	it, that; neuter of onъ; neuter of onъ
2	Proto-Indo-European	h₂en	there

ono — Etymology, Origin & Word History | Etymology Explorer