Superfluous words are best eliminated from our writing, even when they sound perfectly acceptable in colloquial English. The word of, when used in combination with some other words, is sometimes superfluous.
For example, take the use of of in what Claire Cook calls “double prepositions.” Cook writes, “Two prepositions in a row are often one too many” (188). In the following examples, instead of the double prepositions outside of, we can omit of and retain only outside:
writers living outside of the United States → writers living outside the United States
a question outside of his purview → a question outside his purview
Likewise, we can often omit of in the double prepositions inside of, alongside of, and off of:
sitting inside of the car → sitting inside the car
walked alongside of the river → walked alongside the river
the tourists stepped off of the bus → the tourists stepped off the bus
It’s not just double prepositions we should look out for. A superfluous of can crop up in other constructions, too. When the word both is used as an adjective, it is sometimes paired with of. The combination both of can often be reduced to simply both:
both of the players were disqualified → both players were disqualified
both of the flights are overbooked → both flights are overbooked
The same principle applies to the combination all of when all is used as an adjective:
all of those problems can be solved → all those problems can be solved
all of the voting members are present → all voting members are present
However, note that of is retained when a personal pronoun follows the phrases both of and all of:
The principal congratulated both of them.
She invited all of us to her son’s graduation.
Cook, Claire Kehrwald. Line by Line: How to Edit Your Own Writing. Modern Language Association of America, 1985.