Charles Ellicott Commentary Leviticus 14:18

"and the rest of the oil that is in the priest`s hand he shall put upon the head of him that is to be cleansed: and the priest shall make atonement for him before Jehovah." — Leviticus 14:18 (ASV)

He shall pour upon the head. — That which remains of the oil in the hollow of the priest’s hand, after some of it had been sprinkled seven times before the Lord and after some had been put on the several organs of the leper’s body, the priest is to put, not “pour,” upon the convalescent’s head—the quantity left in the hand not being sufficient to pour. The bulk of the log of oil from which the hollow handful was taken was the perquisite of the officiating priests and, like the flesh of the sacrifices, had to be consumed within the sacred precincts.

And the priest shall make an atonement. — According to the canons that prevailed during the Second Temple, it was the act of putting the oil on the several organs, but more especially on the head, that effected the atonement of the restored leper. Failing to do this, even if the prescribed sacrifices had been offered, no expiation was made. The atonement was made for the sins that brought leprosy upon the sinner.