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.