An antonym of omen is aftermath. While an omen is a sign or warning of something to come, an aftermath refers to the consequences or results of an event that has already occurred.
For example, if a dark cloud is considered an omen of a storm, the aftermath would be the damage caused by the storm after it has passed. These terms represent opposite concepts in terms of timing and nature—omens are about foresight and prediction, while aftermaths are about reflection and consequence.