Obliquity of the Ecliptic - the orbital element that determines the amplitude of the seasonal variation of incident solar radiation. The present value is 23°2' to 24°27'.

Obscuration - the term used when the sky is completely hidden by surface-based obscuring phenomena

Obscuring Phenomena - any atmospheric phenomenon, except clouds, that restricts vertical visibility (e.g., dust, rain, snow, etc.).

Occluded Front - the front formed by a cold front overtaking a warm or stationary front and lifting the warm air above the earth's surface

Occluded Mesocyclone - A mesocyclone in which air from the rear-flank downdraft has completely surrounded the circulation at low levels, cutting off the inflow of warm unstable low-level air

Orographic Lift - the lifting of air caused by its passage up and over mountains or other sloping terrain

Outflow Boundary - a storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air

Overcast - sky cover classification describing a sky with 95% or more cloud cover

Overhang - a radar term indicating a region of high reflectivity at middle and upper levels above an area of weak reflectivity at low levels. The overhang is found on the inflow side of a thunderstorm (normally the south or southeast side).

Overshooting Top (or Penetrating Top) - a dome-like protrusion above a thunderstorm anvil; it represents a very strong updraft and, hence, a higher potential for severe weather with that storm. A persistent and/or large overshooting top often is present on a supercell.

Oxygen - in its free form, a colorless, tasteless, and odorless gas in the atmosphere of which it forms about 21% by volume

Ozone - a triatomic form of oxygen that is nearly colorless gas of pungent odor; it is formed naturally in the upper atmosphere, generally between the heights of about 10 and 50 km and it results in the absorption of a large part of the sun's most intense incoming radiation