inside of, outside of, off of

When inside, outside and off are used prepositionally, the preposition of is unnecessary and can therefore be removed.

• Inside (not Inside of) government buildings, smoking is prohibited.
• Please wait outside (not outside of) the office doors; the interviewer will call you in one by one.
• When the actress stepped off (not off of) the stage, she let out a triumphant squeal.

Note that inside of is used colloquially to mean in less than or within when the speaker is referring to untis of time.

• They’ll arrive in Saskatoon inside of (within or in less than) a week.