Begin by cleaning the olives. Rinse them well and remove any dirt, leaves, twigs, and other debris.
Add the olives to a food processor or blender and process them at a very low speed to blend the pulp of the olives without breaking up the pits. (Blending at too high of a speed could damage your blender.)
If using an immersion blender or a blender with a narrow base, the pits may end up getting stuck between the blades. In that case, remove the olive pits in order to blend the pulp well without damaging your blender. (Once you have blended the olives a bit, the pits should be easy to remove from the rest of the olive mush.)
Blend the olives at low speed for several minutes. This will not only blend the pulp more thoroughly but will also give off heat, which will help the olives release their oil.
After blending, strain out the olive pulp and pits from the liquids using either a strainer or a cotton cloth. The resulting liquid will be a mixture of olive oil and water-soluble olive juice.
Separating the olive oil from the olive juice
At first, the liquid will look pretty uniform. Allow it to settle and separate. After some time, the liquid should separate into at least 2 layers with the oil layer floating on top of the juice (water-soluble layer).
Siphoned out the lower layer of liquid using food-grade tubing. If you don't have food grade tubing, you can also use straw to help remove the olive juice. (Discard the bitter juice.)
Filtering the oil
Filter the oil by straining it through several layers of cotton cloth or cheesecloth. This can be done by lining a colander with the cloth and letting the oil filter through, leaving the olive solids in the cloth above.
The oil should be clear now, but you may be left with some juice below it. (The two layers should now be more obvious and easy to distinguish.) Tilt the bottle of olive oil on an angle and use a thin straw to remove the liquid from the bottom of the bottle.
Yield in an estimate and will vary greatly on the olive type used and how you process your oil.
Getting a higher yield
There are several things that you can try doing to obtain a higher yield from your olives.
Heat can be used to extract more oils from the olive paste. To use heat to help extract more oils, cook the olive paste in a pan on the stovetop at low heat. Once that is done for several minutes, place the warm olive paste in a bowl lined with cloth and wring it to extract more liquids from the olives.
Further blending without the pits
Generally, the more you can blend the olives, the more oils you can extract from them. One of the best ways to do that is to remove the pits from the olive mush and then further blend the mixture for several minutes. Once you've done that, you should be able to obtain more liquids from the mixture.
Adding hot water
Once you've removed most of the liquid, you can try to extract a bit more oil by blending the pulp with hot water. If you strain out that liquid, you will likely find that you will obtain a bit more oil floating on top of the extracted water. (You'll have to remove the excess water afterward just as you would remove the unwanted olive juices.)
Do you need to filter the oil?
Filtering the oil will not only make it clearer, but by removing the solids that could be retaining moisture (which could make them susceptible to microbial growth- aka. mold and bacterial growth), you are extending the shelf life of your oil.The filtered-out olive solids can be eaten or spread on breads, but they should be stored in the fridge and used within a few days. (The solids will retain some water, making them susceptible to mold growth.)
For the longest shelf-life, store the oil in a cool, dark place.
How long will it keep?
Olive oil is a relatively stable oil that generally keeps well. It's said to be shelf-stable for up to two years. That said, the shelf life of your oil depends a lot on how well you filter it and if you remove the olive juices well.Ideally, you should only have oil in your bottle, free of olive solids and other liquids.