Now, not everybody believes in omens, and that's totally okay. To me, omens make no sense. A black cat attends to its own business, as does a butterfly, and a rainbow appears when sunlight scatters through raindrops—none of these things have much to do with us, and even less to do with what's happening to us later. Historically, however, people have paid a lot of attention to things they saw as bad omens; so, the word "ominous" means "like a bad omen," or more loosely, "creepy, spooky, or sinister in a way that seems to suggest that something bad will happen."
The word "ominous" is common and powerful. You call something ominious when it fills you with a vague fear. You might describe shadows as ominious.
Or thunder. Or a dark look on someone's face. Here's Patrick, rubbing his hands together ominiously. (Does he have hands?) Anyway, he's smiling ominously. If something gives you chills and makes you worry that terrible things are in store for you, call it ominous.