What is a sentence for omen?
Here is an example sentence with the word 'omen':
An eclipse is often seen as a bad omen back in the ancient times.
Thesauruses and dictionaries do not seem to mention what the antonym of an omen is, but they do mention that and omen is a "sign of something to come." So, if an omen means to predict something that has not happened yet, then the opposite of an omen would either be to predict what has happened long ago or something that has already happened.