Once the brush is free of oil paint, and if you're not going to be using it for the moment, clean it with a soap bar, rubbing it and loading it with soap.
Then, "paint" on your hand using the soap-loaded paintbrush.
In case you forget cleaning your brush and the oil paint is already dry, place the paintbrush in a jar with turpentine for a few hours until the oil paint dissolves itself. Then, clean the brush following the same procedure.