'Of' is an odd word. Not semantically, of course. It is a perfectly cromulent word as far as meaning goes. However orthographically and etymologically it is a train wreck. It is spelled, as you can see, with an 'o' and an 'f', but it is pronounced as 'uv'. It is the only word in the English language in which F is pronounced as /v/1. It is also the twin to the more regularly pronounced word 'off' -- in fact, they are identical twins, and an excellent example of how environmental factors can warp development.
'Of' comes from the Old English æf, meaning 'away', or 'away from'. Over time æf came to exist in two forms; the stressed or emphatic /æf/ was used when it functioned as an adverb ("he went off"), and the unstressed /æv/ when used in the genitive case ("Author of Camelot"). But while these were pronounced differently, for a long time this was not considered an important difference, and did not appear in writing2. In Old English /v/ was an allophone of /f/; while the sounds were usually pronounced differently depending on where they appeared in words or sentences, they were not considered different letters, and were both represented by the letter F.
This started to change after the Norman Conquest introduced a large chunk of French vocabulary into England.
The Old French language introduced the importance of distinguishing /v/ from /f/, as these were distinct phonemes in French. Over the coming years (c.1154-1470) most words pronounced with a /v/ sound gained a V; ofer became over, etc. Unfortunately, this distinction passed over 'of', largely because the of/off split was not widely recognized in a formalized manner until the 1600s. Until then the spelling was fluid, so the exact differences between æf, of, and off were not systematic. This split took place long after the other /v/ sounds had gained a firm toehold in print.
Even though of/off split lagged behind the F/V split, they were both caused by the influx of Old French. Old French distinguishes between
de (of) and
ab- (off). Gradually
Middle English/
Early Modern English translators started to realize that they could no longer always treat these two meanings of
æf/of/off as equal. Our odd spelling of 'of' comes from the happenstance that English was quicker to pick up a new sound than a new meaning.
Footnotes:
1. The slash marks (/v/) are used to indicate that this is a phonetic representation of how the letter or word sounds, as opposed to how it is spelled. I will be using this convention throughout the writeup, as is standard practice when working with phonological transcription. All sounds are written using the International Phonetic Alphabet.
2. We do the same thing in Modern English -- and we do it a lot. We "have a cat", but we "haff to wash the cat". We "opened the door", but we "shopt in the store". This sort of thing drives second graders who are learning to spell absolutely crazy, and then we forget about it.