Augustine of Hippo

(ubi sup.) Or, The oil denotes joy, according to that, God hath anointed thee with the oil of gladness. (Ps. 45:7.) He then whose joy springs not from this that he is inwardly pleasing to God, has no oil with him; for they have no gladness in their continent lives, save in the praises of men. But the wise took oil with their lamps, that is, the gladness of good works, in their vessels, that is, they stored it in their heart and conscience, as the Apostle speaks, Let every man prove himself, and then shall he have rejoicing in himself, and not in another. (Gal. 6:4.)