3 Then, when you were stripped, you were anointed with exorcised oil, from the very hairs of your head to your feet, and were made partakers of the good olive-tree, Jesus Christ. For you were cut off from the wild olive-tree, and grafted into the good one, and were made to share the fatness of the true olive-tree. The exorcised oil therefore was a symbol of the participation of the fatness of Christ, being a charm to drive away every trace of hostile influence. For as the breathing of the saints, and the invocation of the Name of God, like fiercest flame, scorch and drive out evil spirits, so also this exorcised oil receives such virtue by the invocation of God and by prayer, as not only to burn and cleanse away the traces of sins, but also to chase away all the invisible powers of the evil one.
Source: Catechetical Lectures (New Advent)