Work on the first edition began around 1860 under the auspices of the Philological Society. Later the huge undertaking was heavily subsidized by the Oxford University Press. The original 12 volumes...
The OED attempts to be comprehensive, listing literally every word in the English language along with citations for its earliest known use. It's essentially a really really big dictionary.
The longest words in the Oxford English Dictionary are: 44 letters: pneumonoultramicroscopicsilicovolcanoconiosis (a lung disease which nobody ever seems to get - it's suspected that this might be a...