Reconstruction:Proto-Celtic/-os

From Proto-Indo-European *(ó)-os, which formed tómos-type event nouns from verbal roots.^[1]

*(o)-os m

1. Forms verbal nouns corresponding to o-grade causative/iterative verbs.

   ‎*rād-ī- + ‎*-os → ‎*rādos (“speech”)

   ‎*uɸo- (“under-”) + ‎*log-ī- (“to lay”) + ‎*-os → ‎*uɸologos (“hiding”)

This suffix causes the root to assume the o-grade. It can form both unprefixed and prefixed verbal nouns, unlike *-om (which can only make prefixed verbal nouns).