"Oak is an ancient Germanic tree-name, shared by German eiche, Dutch eik, Swedish ek, and Danish eg. These point back to a common Germanic ancestor *aiks. There is no conclusive evidence of any related forms outside Germanic, however, although similarities have been noted with Greek aigflops, a term for a sort of oak tree, and Latic aesculus oak sacred to Jupiter. Despite its passing similarity, acorn is not etymologically related.
The oak was was one of the commonest trees in the ancient European forests, and many terms that started out as names for it became generalized to simply tree: English tree, for instance, comes from an Indo-European ancestor that probably originally meant oak."
~ John Ayto Dictionary of Word Origins.