Where does “-o” come from?

-o (Old Portuguese) comes from Latin -ō, from Proto-Indo-European -ō — Derives nouns from roots.

Ancestry of “-o”, step by step

Step	Language	Word	Meaning
1	Latin	-ō	suffixed to nouns or adjectives. Originally a-stem nouns. Later extended to nouns with other stems. Forms regular first-conjugation verbs
2	Proto-Indo-European	-ō	Derives nouns from roots