But what Alex (a gray parrot) once did outside of the laboratory was even more impressive.

Is "of" in the bolded phrase optional to you? Thanks.
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Comments  (Page 2) 
It is becoming increasingly common to hear 'inside' and 'outside' with the 'of' tag, which is incorrect. The two words function perfectly as prepositions without the help of the extra word.

Both words, however, can be used as nouns: The inside of the house is absolutely filthy, while the outside of the house is neatly painted in a combination of colors.
Philip, I don't remember when you visited my house. - A.
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