Oats   A type of food plant.

Oil   Dead dinosaurs, worth trillions of dollars.

OMNI  The name of a defunct Science/Science Fiction magazine.

Oreo  A very popular snack that sandwiches white cream between two crisp chocolate cookies.  Probably evil.

Outer Space  Everything outside the Earth's atmosphere that is not an object like a rock or a planet or a sun. A very big place, in which the earth is not statistically significant, or existent for that matter.