What was olive oil a symbol of in the Bible?

The oil more generally, and not only in the context of anointing, is also a picture of the Holy Spirit. Consider, for example, the following:

Oil gave light (Ex. 27:20–21; Zech. 4:2–6). The Holy Spirit illuminates and teaches (1 John 2:20, 27). The vessels of the tabernacle were anointed with oil to consecrate or sanctify them for God (Ex. 40:9; Lev. 8:10–12). The Holy Spirit sanctifies (1 Pet. 1:2). The ‘oil in a cruse’ (1 Ki. 17:12) shows the Spirit of God abiding with us forever (John 14:16) — without diminishing over time in any way, even if we share our blessings with others. The vessels that had to be filled with oil (2 Ki. 4:2ff) show that believers should be filled with the Spirit. The ‘Good Samaritan’ poured oil in the wounds of the man who had fallen among thieves (Luke 10:34). This oil was for healing and overcoming the effects of sin. It is by the Spirit that the righteousness of the law is fulfilled (Rom. 8:4) and the activity of the flesh is subdued (Gal. 5:16). The ‘oil of gladness’ (Ps. 45:7) speaks of the joy in the Holy Spirit (Rom. 14:17). The ‘oil to make his face to shine’ (Ps. 104:15) reminds us that, ‘looking on the glory of the Lord, with unveiled face’, we are transformed by the Spirit (2 Cor. 3:18). The ‘holy oil’ (Ps. 89:20) reminds us that the Spirit is holy and makes holy.

The oil more generally, and not only in the context of anointing (Q.41), is also a picture of the Holy Spirit. Consider, for example, the following:

Oil gave light (Ex. 27:20–21; Zech. 4:2–6). The Holy Spirit illuminates and teaches (1 John 2:20, 27). The vessels of the tabernacle were anointed with oil to consecrate or sanctify them for God (Ex. 40:9; Lev. 8:10–12). The Holy Spirit sanctifies (1 Pet. 1:2). The ‘oil in a cruse’ (1 Ki. 17:12) shows the Spirit of God abiding with us forever (John 14:16) — without diminishing over time in any way, even if we share our blessings with others. The vessels that had to be filled with oil (2 Ki. 4:2ff) show that believers should be filled with the Spirit. The ‘Good Samaritan’ poured oil in the wounds of the man who had fallen among thieves (Luke 10:34). This oil was for healing and overcoming the effects of sin. It is by the Spirit that the righteousness of the law is fulfilled (Rom. 8:4) and the activity of the flesh is subdued (Gal. 5:16). The ‘oil of gladness’ (Ps. 45:7) speaks of the joy in the Holy Spirit (Rom. 14:17). The ‘oil to make his face to shine’ (Ps. 104:15) reminds us that, ‘looking on the glory of the Lord, with unveiled face’, we are transformed by the Spirit (2 Cor. 3:18). The ‘holy oil’ (Ps. 89:20) reminds us that the Spirit is holy and makes holy.

The word, anointing, as a symbolic word carries with it a definite meaning. There were three classes anointed in Old Testament times—priests, prophets, kings. Of course they were anointed by oil being poured upon their heads. But though it was a rite in Old Testament times, it carried with it a definite result. This is plainly indicated in 1 Samuel 16:13: “Then Samuel took the horn of oil and anointed him [David] in the midst of his brethren: and the Spirit of the Lord came upon David from that day forward.” Scripture thus links anointing with oil with the bestowal of the Spirit. With the priest, it was to enable him to take the priestly position and service; with the prophet the prophetic position and service; with the king the kingly position and service.

Oil in the Old Testament is typical of the Holy Spirit. In the New Testament the Holy Spirit is called the “unction” (1 John 2:20), and the “anointing” (1 John 2:27). In this beautiful Psalm we get an illustration of the pouring out of the Holy Spirit in the ointment descending from Aaron’s head, down to his beard, and thence to the skirts of his garments. The Spirit of God descended from the Head in heaven to indwell all the members of Christ’s body on earth. Descending to Aaron’s beard may set forth the way that our Lord conferred the Spirit first on His apostles, and then the Spirit being bestowed upon every believer answers to the precious ointment reaching the skirts of Aaron’s garments.

Oil is clearly a type of the Holy Spirit. It was called the anointing oil. The Spirit as given to the believers is called “the anointing.” “The anointing which ye have received of Him abides in you, and ye need not that any man teach you” (1 John 2:7).

God often speaks to us by symbols, types, emblems. He uses the seal when telling us of our security in Christ (Eph. 1:13.14). When God would reveal to us the healing, comforting, illuminating, and consecrating influences of the Sprit, He directs our minds to the oil, which shadows forth these characteristics.

The oil, as we all are happily aware, is a type of the Holy Spirit of God in power. In the Old Testament we have the Spirit in the figure of running water; seen there as indicative of refreshment; again we have the Spirit as fire in maintaining judicially the rights of God. There are many other figurative references to the Spirit, but the typical bearing of "oil" is the Spirit in divine power operating in this world for God. As coming upon persons, it is in order to give them spiritual power to carry out the will of God; a feature seen in perfection in the Lord Jesus Christ.

The oil is typical of the Holy Spirit of God in power. In the Old Testament the Holy Spirit is typified by several things. Firstly, living water, along the line of refreshment; sometimes He is typified by fire in judicially maintaining the rights of God; sometimes He is portrayed by light (as seen in the lampstand in the tabernacle), and there are others. However, oil typifies the Spirit in divine power operating in this world for God. When He came upon people it was to give them spiritual power to carry out the will of God. This was perfectly performed in the Lord.

The olive oil, which God commanded the children of Israel to bring to Moses, to cause the lamps to burn continually, is a well-known symbol of the Spirit of God. There would be nothing in this world for God at the present time if the Holy Spirit was not here; and every bit of light that shines in the lives of the saints, both for the pleasure of God, and for testimony to the world, is the fruit of the Spirit's work in the hearts of the saints of God.

The chapter closes with the direction that "pure oil olive beaten" be brought to cause the lamp in the holy place to burn continually. Only twice before have we read of oil, both times when Jacob poured it on a pillar (Gen. 28 and Gen. 35) but there the nature of the oil is not specified. Here its nature is specified. It was to be beaten out of the fruit of the olive and pure. This first mention of pure olive oil is clearly typical of the Holy Spirit, and fixes its significance right through the Scripture. The golden lampstand would have been of no service without the oil. In the New Testament the churches, as well as the individual believer, are likened to lamps. But apart from the oil of the Holy Spirit they have no ability to shine to the glory of God.