Opportunity cost

Hi john sir..how r u..
john sir i read in an artical that all opportunity cost are notional cost but all notional cost are not opportunity cost..my question is that if all opportunity cost are notiinal then how can we treat opportunity cost relevant while making decison using relevant costing approch..moreover i read somewhere that..depreciation is a real cost..is it true? If it is true then why we do not consider it relevant cost in the relevant costing approach