A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Oath of Office
OPEC
Ordinance
Organization of the Petroleum Exporting Countries
Original Intent
Oval Office
Ozone
Ozone
Ozone is a molecule made up of three atoms of oxygen. It occurs naturally in the stratosphere and provides a protective layer shielding the Earth from harmful ultraviolet radiation. Below the stratosphere, in the troposphere, it is a chemical oxidant, a greenhouse gas and a major component of photochemical smog.

(Courtesy of Energy Information Administration, U.S. Department of Energy)