Extraction
The olive paste contains a percentage of oil varying from 18% to 27%, which can be extracted with different methods. 
The yield, that is to say the quantity in weight of oil that can be extracted from 100 kg of olives, depends on the type of olives, on the amount of water received by the plant, on the fertilizing, on the harvesting system, and above all on the extraction technique.
 

Yesterday
The oldest extraction plants were based on lever presses in which heavy beams crushed the olive pulp. In recent centuries, vertical screw presses were introduced. They allowed the application of pressure on the olive paste inside containers made of intertwined vegetal fibers, carefully stacked in the press. When the pressure plate or beam was lowered, the oil oozed out of the containers, to be gathered inside vats or collection pits.
To facilitate the flow and to avoid its becoming dense because of cold weather, the mill temperature was kept constantly high, and ladles of hot water were poured on the stacked containers.
In time, the same procedure was maintained as presses were introduced that were made totally or partially with metal. At the turn of the 20th century hydraulic presses were introduced. 
Wooden screw press - Western Liguria, 19th century
The picture shows a hand-operated press with containers in position.

Today
The most recent techniques have radically changed the oil extraction concepts and methods. In modern olive mills extraction from the olive paste is based on the principles of centrifugation and a process called "percolation" (not to be confused with the coffee brewing process).

Centrifugation 
This process exploits the specific weight differences between water and oil. The water-thinned paste is churned at very high speed in a horizontal centrifuge. This results in the separation of the three components of the paste: oil, vegetation water and the pit and pulp residue (pomace). 

Percolation 
This method of separation is based on the different surface tensions of oil and water. We can understand how it works by dipping a metal blade, such as a table knife, into a glass of oil and water. When we pull out the blade we will note that only the oil has adhered to it. Thus percolating machines have been designed to dip numerous steel blades, attached to a continuously moving belt, into a receptacle containing the water-thinned olive paste. As the blades move out of the water their load of oil drips off into a collecting basin. This technique allows the extraction of about 50%-70% of the oil in the solution and results in an excellent product, free of any contaminants. The oil not picked-up by the percolation process is then recovered from the remaining oil and water solution by centrifugation.
Centrifuges
The picture shows two vertical centrifuges used in the final processing phase to separate any remaining drops of water from the oil.