Of for have
Of is sometimes written by mistake instead of have, in phrases such as could of, should of, would of. It happens because the auxiliary verb have is often contracted in spoken English to ‘ve, which sounds the same as of. It should be corrected to have in formal writing, or otherwise written as ‘ve to represent nonstandard speech:
We could have gone to the party. She should have told us.
We could of gone to the party. She should of told us.