Mythology
Oak groves, or nemetons, have significance, in that the word Duir/oak means 'door', having its roots in dorus (door), as does druid. A druid is essentially and by definition therefore the 'doorman' or guardian of the (oak) door - the door that looks both ways - which makes sense when it is appreciated that the Duir/oak ends the first half of celtic year at midsummer solstice. Traditionally, Nemi (the 'oak-king') was sacrificied (and replaced) yearly on this date. A similar figure in Roman times is Janus, who is usually illustrated with two heads that look either way. Another associated goddess is Cardea, whose significance belongs more to Huath/hawthorn, and is related to the door-hinge. Although sounding rather a pedantic purpose for divinity, her significance lies in the fact that Cardea is 'the celestial goddess at back of the north wind' who 'calls to the door'. This door, and therefore her vision, leads to reincarnation or other-world.
Like Zeus, Jupiter, Thor, Lugh (the 'Shining One') and the King of Nemi
('Oak-King'), all oak gods and guardians are entitled to thunder (or use
the lightning bolt) and in the Welsh poem Battle of the Trees (Cad Goddau)
his connection as door between worlds is made:
Before him tremble heaven and earth, stout doorkeeper against the foe, is his
name in all lands.
The sacred oak at Dodona (Greece) was, like that of Delphi, a major oracle of the Old World, and an international place of pilgrimage for nearly a thousand years. Its messages were interpreted from the rustling of its branches, from the murmur of the sacred spring that welled from beneath its roots, and thirdly through selection of one of the many oracle lots kept in an urn beneath it. Two priestesses of Dodona asserted that the oracles were brought by two black pigeons from Thebes which (like the craneskin in which the alphabet was brought to Greece by Palamedes), suggests a constant migratory theme that remains merely hinted at, yet quietly insistent in the background.
Added notes: in Egypt oak has origins in Amon Ra (the ram) an eight-rayed sun-disc in hand and spiral of immortality sprouting from his head; mistletoe grows on oak (and hence dwells between heaven & earth). Associations: coltsfoot, white horse, lion, salamander