The olive tree from ancient times was a symbol of peace and reconciliation. The flourishes of olive tree in the eastern countries around the Mediterranean and especially in the holy places defined it as a symbol in the Christian religion. There are many references in the Gospels and the Psalms as you will see below.

There were olive trees, at the Mount of Olives and the Garden of Gethsemane, the places where take place the events of the Passion of the Christ (Mt. 26: 30-38).

The fruiting of olive tree likens the rich blessings of God in the lives of believers (Psalms 51: 10).

The olive tree rich in fruiting symbolize a mother which enriches her home with her children, the fruits of her love (Psalms 127: 3).

The apostle Paul likens the unfaithful human as "agrielaio" (wild olive tree) and the Church to tame olive tree. When the wild olive branch, grafted to tame olive tree acquire the attributes of the tame tree, so the human becomes "kallielaios" (tame olive tree) when be part of the Church (Romans 11: 16-24).

Symbolic is the olive oil too. In the Bible it is presented as a symbol of joy, because the human who  smears his face  with olive oil shining (Psalms 103: 15).

The Good Samaritan treat with oil the wounds the beaten traveler (Lk. 10: 30-37).
Also olive oil used in the sacrament of baptism and religious unction. Especially on Holy Wednesday in St. Unction mystery, the priest reads seven Gospels and seven wishes to bless the olive oil that crosses the believers to treat mental and physical illnesses.