hey there! The Omen score is in some ways a shifting difficulty class score for each character. If a PC has an omen of 55 and they roll 30, they succeed on their roll and their omen score becomes 30. The next time they roll they would be going for under 30, so if they rolled an 89 they would fail, but their omen would become 89, giving them a pretty good chance at success next time. Repeat forever.
Hopefully that helps a little bit!