Most words of the former class have -us (Latin) or -ος -os (Greek) in the nominative singular, except for the r-stem nouns in Latin, and the "Attic" declension and contracted declension in Attic Greek (when these groups are considered part of this declension).
In most cases, these words end with -us in Latin or -ος (-os) in Greek, with exceptions for r-stem nouns in Latin and specific declensions in Attic Greek.