In a small bowl, gently whisk the witch hazel with the olive oil and castile soap until combined.
Hold the brush bristles at a downward angle and immerse the bristles into the cleaning solution, turning to coat. Allow to soak for 5 minutes.
Remove the brush from the cleaning solution. Allow any excess to drip away. Gently swipe the brush back and forth on a clean towel.
Rinse the brush under lukewarm running water until the water runs clear. Repeat the process, if necessary, until the brush is clean.
Lay brush flat on the counter with the bristles hanging over the edge to air dry. Alternatively, lay brush flat on a clean towel to dry, rotating occasionally.