Depends on how you use it. If you are explaining something, you might use the phrase 'of course' and it is not offensive. You could use it as a remark to make someone feel stupid for something they said, so in that case, it might be seen as offensive.

No, what makes you think that it is?

as far as i know it won't be offensive as long as u r use it with proper tone

I agree with the advice you have already been given, but, just to confuse things, there are times when 'of course' is an entirely appropriate response because it can emphasise your sincerity. Say you ask a friend to go to a party with you, and they are not sure if you really mean it, so they say "are you sure you want me to come too?", and you reply "of course", to emphasise that yes, that really is what you meant.

It's not an offensive expression in itself, but it can cause offence if used inappropriately. If I make a statement, or ask a question requiring confirmation, and you respond by saying 'Of course', this can be seen as insulting. For example, 'Is this true?' 'Of course.' The response 'Of course' implies 'Of course it's true. This is obvious.' (And how stupid you are to ask). The person who told you to avoid answering 'Of course' gave you some good advice.