====== O ======
===== Octane =====
The measure of a fuel's resistance to knock or ping  ((Haynes Sportster manual 1970-2010 pg REF*50))
===== OD =====
Abbreviation for __Outside Diameter__
===== OEM =====
Original Equipment Manufacturer - OEM items are parts that are designed and built by the vehicle's manufacturer and specifically for their vehicles. Harley-Davidson® is the OEM for all Sportster parts.
===== Ohms =====

===== Ohm Meter =====
An instrument used for measuring electrical resistance. See also Multimeter
===== Oil Cooler =====
A system of bringing the oil to the outside of the motor to receive cooler air flow over it then it's returned back to the engine
===== Open Circuit =====
Condition in which there is a break in the electrical current. No continuity (high resistance) ((Haynes Sportster manual 1970-2010 pg REF*50)) 
===== Overall Gear Ratios =====
Represents the number of engine revolutions to turn the rear wheel one full turn.
===== Overrun =====
Shutting the throttle at high engine speed (which is the common time to getting exhaust popping). When this happens, the engine sucks hard and the throttle isn't open much which results in a low pressure in the manifold ((FoxsterUK of the XLFORUM http://xlforum.net/forums/showthread.php?p=5625387#post5625387)) and a high vacuum in the venturi.((IronMick of the XLFORUM http://xlforum.net/forums/showthread.php?p=5625387#post5625387)) The high vacuum on overrun will be enough to slow the throttle's closure. 

===== Oxygen Sensor =====
Installed in the exhaust system to allow monitoring of the oxygen in the exhaust flow and converted information sent back with an electrical current




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