Are you asking about the pronunciation* of the words 'off' and 'of'? In other words, how to say these words?
'Off' is pronounced with a f sound at the end ( like the sound at the beginning of 'fun') .
'Of' is pronounced with a v sound at the end ( like the sound at the beginning of 'voice').
It's also worth mentioning that 'of' is usually unstressed in a sentence, so the the 'o' is reduced to a schwa, which is a light 'u' sound. This means that when we say, 'cup of coffee', for example, the word 'of' sounds like a 'uv'.
* pronunciation = how we say words
spelling = how we write words
I hope that helps.