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Of (ɒv) is a preposoition used to show possession or the relationship between two things.

For example:-

The Queen of England is a friend of mine.

Off (ɒf) can be used as an adverb to show when something is away from a certain place or time, or as a preposition of movement.

For example:-

Adverb - I accidentally scratched the door of the car.

Preposition - The ball rolled off the table.

It can also show if something is no longer fresh.

For example:-

I'm not putting this milk in my tea, it's off.

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