Where does “ó” come from?

ó (Galician) comes from Galician o, from Vulgar Latin illu, from Latin ille, from Latin olle, from Proto-Italic *olnos — that.

Ancestry of “ó”, step by step

Step	Language	Word	Meaning
1	Galician	o	the; accusative of el
2	Vulgar Latin	illu	the
3	Latin	ille	that; those; "; demonstrative pronoun "; that...
4	Latin	olle	Archaic form of ille
5	Proto-Italic	*olnos	that