Holders of the priesthood had numerous responsibilities during Old Testament times. Many of those duties are outlined in Leviticus and other early Old Testament books.
In Unlocking the Old Testament, Victor L. Ludlow explained the different priesthood offices and outlined many of the specific priesthood responsibilities."The high priest (Aaron and his male descendants), priests (all male descendants of Moses and Aaron), and Levites (other male descendants of Levi) had many duties in ancient Israel. Some responsibilities were restricted to particular offices, while most of them were done jointly. Among them were:
"1. Use Urim and Thummim (Ex. 28:30; Num. 27:21); high priest only.
"2. Administer worship in the tabernacle (or later in the temple: Lev. 24:5-9; Ex. 30:7-10, 22-38); high priest and priests.
"3. Serve as religious judges in Israel (Lev. 13, 14; Num. 35:6-32); high priest and priests.
"4. Care for sacred articles of tabernacle or temple (Num. 4:5-20); high priest and priests.
"5. Supervise circumcisions, washings, and baptisms (Ex. 40:12; Josh. 5:8; D&C 84:26-27); priests.
"6. Assist in sacrifices (Lev. 6:12; 9:13; Ex. 29:38-44); high priest (Day of Atonement), priests, and Levites.
"7. Teach Israel the laws and covenants (Lev. 10:11; Deut. 33:10); all offices.
"8. Collect and distribute tithes and offerings (Lev. 9:16-22); all offices.
"9. Move tabernacle and maintain tabernacle and fences (later temple and courtyards: Num. 3:5-13; 23-37; 4); all offices.
"10. Blow the silver trumpets to announce religious festivals and holy days or to call men to war (Num. 10:1-8); Levites.
"11. Work in all types of service at the tabernacle or temple (I Cor. 6:48); Levites."
In Studies in Scripture: The Old Testament, Kent P. Jackson noted, "It should be understood that the Aaronic Priesthood in the Church today is the same priesthood as that of Old Testament times. It was restored to Joseph Smith by John the Baptist, the last holder of the keys to that priesthood under the Mosaic dispensation.
"Yet it functions under an entirely different system today and fills a different purpose. The ancient Aaronic Priesthood system was the hereditary priesthood hierarchy of Israel under the Law of Moses. Our Church today, in contrast, is presided over by the Melchizedek Priesthood and enjoys the fulness of priesthood authority, blessings, and gospel understanding that God makes available to His children. We enjoy today what righteous Saints prior to Moses also had, and what Israel in the Mosaic dispensation could have had if they had remained worthy."