Oleic acid is a fatty acid found in animal and vegetable oils.  It is called a mono-unsaturated fatty acid because of the single double bond between the carbons.  It's physical properties are determined by the number, geometry, and position of this double bond and the degree of unsaturation.
 
 

    Oleic acid occurs naturally in greater quantities than any other fatty acid.  It is present as glycerides in most fats and oils.  High concentrations of Oleic acid can lower blood levels of cholesterol.  It is used in the food industry to make synthetic butters and cheeses.  It is also used to flavor baked goods, candy, ice cream, and sodas.

Figure 2.  The olive tree is a major sources of Oleic acid.

Figure 3.  Oleic acid is also present in the cocoa butter of chocolate.
 
 

References:
Merck Index, Tenth edition
Olive tree photo from  http://www.ch.ic.ac.uk/mim/life/html/oleic_text.htm
Chocolate photo from  http://www.chocolate.com
A more complete discussion of Oleic acid can be found at  http://www.ch.ic.ac.uk/mim/life/html/oleic_text.htm