In the eastern Roman Empire the use and importance of olive oil in the cultural and economic life continued following the fall of Rome. This was particularly true under Constantine the Great. Some Christians believed that three different types of wood were used for the cross of Jesus with one of those being olive wood.

The Christians in the eastern empire had perfect climatic conditions for growing olives in some regions, particularly in some part of Greece and Asia Minor. In Greece, some locations were known during this period for its olives and olive oil. These included Salonika, Thrace, Macedonia, and Thessaly as well as the region along the Aliakmon and Axios Rivers and the Aegean region between Turkey and Greece. In Asia Minor, olive production was also good in the valley of western Asia Minor as well as in Bithynia, which was a Roman province and kingdom in northwest Asia Minor and in Pontus. The latter was a strategic province on the south shore of the Black Sea in northeast Asia Minor.

Olive oil played a role in the early Christian sacraments. It symbolized the works of the just and the grace of God and the Holy Spirit. This also represented mercy, charity, sweetness, strength, and spiritual activity. In the Roman Catholic Church, this was used for sanctuary lamps and for anointing.

Apparently from about the 6th century to about 1300 A.D. the climate of northern Europe and England was at least several degrees warmer than it is at the present time. As a result, some experts believe the growing season could have possibly been a month longer. This meant that people in some areas that are colder now could indeed have grown olive trees at that time.

During the 12th and 13th century A.D., the olive tree was the most widely grown tree in the Mediterranean region. They were cultivated in all regions of Italy. Each region had its own varieties that were suited to the local growing conditions. By 1300 A.D., olive oil was considered a staple in the diet in the Mediterranean region.

One of the monastic orders of the Roman Catholic Church has historical ties to the olive tree for the plant appears on its emblem. The Olivetans were founded in 1313 at Monte Oliveto or Accona. This area was named for the many olive groves. The group was founded by a Sienese nobleman, St. Bernard Tolumel, who became a hermit and retired with two companions in the area.

As far back as Moorish times, the Andalusian region of Spain was known for its premium quality olive oil. This continued during the Middle Ages. Both Pliny and Columella wrote about this in the first century A.D.

The Medici family in Tuscany, Italy promoted the cultivation of the olive tree. This was done partly by selling land cheaply to those who would grow only olives and grapes. The campaign was a success for the area was exporting oil by the 1500s.