The first and third are pure olive oil; the chrism contains an admixture of balm or balsam. Each of them and their uses are described under their own titles.
The use of oil for the administering of Sacraments undoubtedly goes back to the beginning of the Church (James 5). Gradually it was employed also for various consecrations and blessings. Its use is mentioned frequently in the Old Testament. The blessing of oils for each diocese takes place on Holy Thursday in the cathedral church, near the end of the Mass. It is given by the bishop, who is attended by twelve priests wearing priests‘ vestments, seven vested as deacons, and seven others in the garb of subdeacons, besides other ministers. This practice goes back to about the 6th century.
Holy oils are preserved in metallic bottles, kept in an ambry or locked box affixed to the wall of the sanctuary. The oil of catechumens is labeled O C or O S (oleum catechumenorum or oleum sanctum); the holy chrism has the letters S C (sanctum chrisma); and the oil of the sick has I O (oleum infirmorum). The O S and the S C are kept also at the baptismal font, to be used in the ceremonies of Baptism. For sick-calls, the oils are kept in a metal case known as an oil-stock. After Holy Thursday of each year the unused oils are burned in the sanctuary lamp, like ordinary oil. Oil, because it is a food and a remedy, is symbolic of spiritual nourishment and curing; because it gives light, it signifies the light of grace; because it may be used as a liniment, it denotes the giving of strength. The balm in the holy chrism is symbolical of the sweet odor of virtue.