You confused us with the word 'mention' - we didn't know what you were asking. You should have said - for example -
What is the difference between the pronunciation of 'of' and'off' ?
What is the difference between how we SAY or PRONOUNCE 'of' and 'off'?
And - as Jmat has said - 'off' has a /f/ sound, while 'of' has a /v/ sound.