The olive tree is among the oldest known cultivated trees in the world. It originated on the eastern Mediterranean coast, and spread to the rest of the Mediterranean basin 6,000 years ago. The use of oil is now found in many religions and cultures, and there are thousands of varieties of olives. About 12 dozen olive varieties account for 85% of the world olive crop today.
There are many sources for further information about the history and production of olive oil. Here are two. See the Olive Oil Source and article by University of California Cooperative Extension olive oil specialist Paul Vossen.