long or short o?

I was told that the Masculine nominative singular -os in Gaulish
was pronounced long rather than short, while in Greek the nominative
ending -os is pronounced short.

However, if the Gaulish -os is pronounced long, then how come
whenever Gaulish words are written in reference books (Mythology,
etc.), a macron is never placed over this nominative -os ending to
indicate its long? A macro is always placed over another vowel, but
never -os.

-Michael