Off ofWhy you should avoid using "off of"
The expression off of is quite common in colloquial English, but it is considered nonstandard. Off is a preposition that does not need the redundant addition of another preposition (of) to express this meaning:
fell off of the shelf
fell off the shelf
Common Errors in English Usage: Errors in diction and idiom commonly made by native speakers of English
List of Common Errors in English Usage (PDF): Printable version of the complete list
Common Grammar Errors: A list of common errors in grammar (topics like subject-verb agreement and parallelism) as distinct from usage