Definition of oil spot:
(n) :
A soiled spot on clothing, a tablecloth, etc., where some oil was absorbed; a similar spot on a driveway, puddle, etc., where some oil is lying.
(n) :
A spot consisting of oil.
(n) :
An oil film over an area of the ocean after an oil spill.
(n) :
(botany) One of the yellow spots, often circular in shape, on the upper leaf surface of grapevines in the early stages of downy mildew (caused by Plasmopara viticola infection).