"tree or shrub of the genus Quercus," Middle English oke, from Old English ac "oak tree" and in part from cognate Old Norse eik, both from Proto-Germanic *aiks (source also of Old Saxon and Old Frisian ek, Middle Dutch eike, Dutch eik, Old High German eih, German Eiche, Swedish ek, Danish eg), a word of uncertain origin with no certain cognates outside Germanic.
The usual Indo-European base for "oak" (*deru-) has become Modern English tree (n.). In Greek and Celtic, meanwhile, words for "oak" are from the Indo-European root for "tree." All this probably reflects the importance of the oak, the monarch of the forest, to ancient Indo-Europeans. Likewise, as there were no oaks in Iceland, the Old Norse word eik came to be used by the viking settlers there for "tree" in general.
In English the word is used in Biblical translations to render Hebrew elah (probably usually "terebinth tree") and four other words. The form in Middle English was very uncertain (oc, oek, hokke, ake, eoke, aike, hock, etc.). Oak-gall "excrescence produced by an oak tree in reaction to insects," used in making ink, is by 1712.