What is the difference between "could've" and "could of"? [duplicate]

"Could of" is always wrong.

It's just that the contracted "have" in "could've" sounds like "of", so uneducated people started writing it as "of" too.

(For the record, "should of", "would of", "might of", and the like are also always wrong.)

Technically speaking, the phrase "could of" (in this context) arose from a mishearing of the contraction "could've" (as Hellion points out) and the literal definition of the words does not convey the intended meaning behind the phrase "could have". This is called a malapropism: "the mistaken use of a word in place of a similar-sounding one" (from Wikipedia).

With that said, language is not a set of "right" and "wrong". It evolves as our habits do, and things that were once considered "wrong" can become the accepted way of speaking and writing, even in the most formal settings. For example, the word "apron" used to be spelled "napron" -- but because the phrase "mine napron" was so common, the word was misheard so frequently that it eventually changed. That and several other examples of malapropisms that entered the English language are written about in this guardian article.

Recent printed usage of "could of" is nothing compared to "could have". In professional and formal settings, using the phrase "could have" is definitely preferable.