I think that the confusion people make between of and off is a simple typing or sound one, as most people understand the difference between these two prepositions.

Of expresses a relationship between a part and a whole, “half of the pie”; a scale/measure and a value, “2 metres of fabric”; an association between two things, usually of belonging, “the sister of the man involved”; a direction and a point of reference, “East of the clock tower”; any other relationship, e.g. “half of you come from Outer Mongolia”, “you, the people of Outer Mongolia”, “Of all the countries in that area, Outer Mongolia is the largest”, “the dress is made of Outer Mongolian fabric”.

Off is used as an adverb or preposition and implies the opposite of a relationship – a separation: away from the place in question “off-site data storage”; so as to be removed or separated from, “the top of the bottle comes off for easy storage”; starting a journey or race, “they’re off!”; moving away and often down from, “she got off the horse”; removed or separated from, “he is now off the Mongolia project”; having a temporary dislike of, “I’ve gone off yoghurt again”; bad or spoiled, “this meat is off”.

So they’re quite different, and it’s just important to watch your typing when you’re using one or the other.

Interesting points 1 – it is incorrect to use of instead of have in constructions such as “I should have asked him the way to Ulan Bator” – see my post on would have or would of for why this happens.

Interesting points 2 – it is also incorrect to use off of instead of of – “I picked him up off of the floor and dumped him in a taxi” – but this was used in Shakespearean English and is found in American English usages (usually not formal written English, though).

You can find more troublesome pairs here and the index to them all so far is here.