In Greek mythology, the oak is the tree sacred to Zeus, king of the gods. In Zeus's oracle in Dodona, Epirus, the sacred oak was the centerpiece of the precinct, and the priests would divine the pronouncements of the god by interpreting the rustling of the oak's leaves.

In Baltic mythology, the oak is the sacred tree of Latvian Pērkons, Lithuanian Perkūnas and Prussian Perkūns. Pērkons is the god of thunder and one of the most important deities in the Baltic pantheon.

In Celtic polytheism, the name of the oak tree was part of the Proto-Celtic word for 'druid': *derwo-weyd- > *druwid- ; however, Proto-Celtic *derwo- (and *dru-) can also be adjectives for 'strong' and 'firm', so Ranko Matasovic interprets that *druwid- may mean 'strong knowledge'. As in other Indo-European faiths, Taranus, being a Thunder God, was associated with the oak tree. The Indo-Europeans worshiped the oak and connected it with a thunder or lightning god; "tree" and drus may also be cognate with "Druid," the Celtic priest to whom the oak was sacred. There has even been a study that shows that oaks are more likely to be struck by lightning than any other tree of the same height.

In Norse mythology, the oak was sacred to the thunder god, Thor. Thor's Oak was a sacred tree of the Germanic Chatti tribe. According to legend, the Christianisation of the heathen tribes by Saint Boniface was marked by the oak's being replaced by the fir (whose triangular shape symbolizes the Trinity) as a "sacred" tree.

Thrice on my bossy shield I struck my spear;

And thrice a ghost's shrill voice was heard in air;

The sacred oaks that skirt this sloping wood

Are dead--revive their withered roots with blood;

The blood of foes shall fertilze the plain,

and Odin's spirt feast on heaps of slain.

Hark! now I hear his mighty voice from far--

Rise, sons of Odin, and prepare for war

In the Bible, the oak tree at Shechem is the site where Jacob buries the foreign gods of his people (Gen. 35:4) . In addition, Joshua erects a stone under an oak tree as the first covenant of the Lord (Josh. 24.25–7). In Isaiah 61, the prophet refers to the Israelites as "Oaks of Righteousness".

The badnjak is central tradition in Serbian Orthodox Church Christmas celebration where young and straight oak, is ceremonially felled early on the morning of Christmas Eve.

In Slavic mythology, the oak was the most important tree of the god Perun.

Source: http://en.wikipedia.org/wiki/Oak