The ancient Indo-Europeans could have been called "The People of the Oak." Because of their reverence for this tree and their dependcy on it many religious associations developed.
Oak trees were sacred to the deities Taranis Indra Jupiter Yahweh Ukho Rhea Kybele Thor Artemis Brigid Balder The Erinyes and the Kikonian Maenads. Perun a Slavic god associated with oak trees was worshipped on the summits of hills and mountains. Parjanyah a Vedic oak god was associated with rain clouds and storms. The Old Norse god Fjorgyn gave birth to Thor associated with oaks who helped his father by slaying his foes with a stone hammer. The Lithuanian god Perkunas was worshipped in sacred oak groves or in the guise of a single oak tree usually growing in a high place. The Greeks had groves of oracular oaks such that in Dordona where the will of Zeus could be heard through the rustling of the leaves.
Among the Balts oakwood fires were kindled in sacred oak groves to honor the high gods and the gods of thunder. The Teutons burned oak fires in honor of the thunder god. For them harming an oak tree in any way was punishable by torture. In 1156 a bishop visiting Lubeck saw a grove in which there was an oak tree surrounded by a fence of stakes. The tree was dedicated to the god of the land.
Tradition states that acorns kept in the home or carried on your person will bring good luck. An ancient Welsh tradition held that good health could be kept by rubbing one's hand on a piece of oak on Midsummer Day while keeping silence. The dew under oak trees was held to be a magical beauty aid.
Oak is above all a symbol of balance. Its roots go as deep as the tree grows high. Oaks are providers - they give of their excellent wood and fees the people and the animals with their nuts. They attract the attention of the highest gods as evidenced by their tendency to attratct lightning yet they are not blasted by the stroke. They have their heads in the clouds and their roots firmly planted in the earth. For Druids oaks symbolize the ideal way of life. /// Source: A Druids Herbal of Sacred Tree Medicine by Ellen Evert Hopman