1) use (ω) at ending of verb ( look=κοιτάζω),and use (ο) at ending of nouns(male and neutral) (human=ανθρωπος,bicycle=ποδήλατο) 2)When we have accent in the third syllable from the end (stand,up = συ-κώ-νο-μαι)

As to the second question, the omega and omikron sound exactly the same in contemporary Greek,"o" sound. In ancient Greek, however, the omega is believed to have sounded twice as long as the omikron. Ωμέγα, ω μέγα (μακρό) και όμικρον, ο μικρόν, the long "ω" (double time) and short "o". This is a language with a lot of history, it's part of its evolution!

3) all verb has ending (-ωνω) we use (ω)