The preposition/adverb, outside, and the compound preposition, outside of, both have the same meaning: beyond the boundaries/limits of _
The New Oxford American Dictionary provides an excellent exposition on the usage of these two:
Outside of tends to be more commonly used in the US than in Britain, where outside usually suffices, but, like its cousin off of, it is colloquial and not recommended for formal writing. … The adverb outside is not problematic when referring to physical space, position , etc. (I‘m going outside), but the compound preposition outside of is often used as a colloquial (and often inferior) way of saying except for, other than, apart from (outside of what I just mentioned, I can’t think of any reason not to). Besides possibly sounding more informal than desired, outside of may cause misunderstanding by suggesting physical space or location when that is not the point to be emphasized, or when no such sense is intended — consider the ambiguity in this sentence: outside of China, he has few interests. Does this mean that his primary interest is China? Or does it mean that whenever he is not in China, he has few interests?