The -o of Romance languages results (mostly) from the Latin second declension, which houses both masculine and neuter nouns. The -o itself is the product of a mixture of the different oblique case endings for the Latin 2nd declension, which ended different depending on the function of the word in the sentence. The -us denotes a nominative masculine case (mostly), while an -ō in Latin, for nouns, can denote a dative or ablative singular masculine/neuter case. As you can see, it gets complicated. The Romance languages do not, for the most part, preserve the inflected case system of Latin and so the "endings" get gradually collapsed, which in Italian and Spanish tends to be -o for masculine and -a for feminine.