The 'Omen' isn't based on real events. It's a work of fiction that plays on common horror tropes and themes to create a suspenseful and terrifying cinematic experience.
The significance of the first omen is that it often serves as a warning or a sign of things to come. In the context of a true story, it might be the initial indication that something big was about to happen. It could be a wake - up call for the people involved, whether they recognized it as such at the time or not.