The opportunity cost is the actual cost of any activities, because it gives the opportunity to see, which is the cost that is hidden (and not be able to see if the accounting calculation).
The opportunity cost is the actual cost of making any event because it represents an opportunity cost that is hidden. (And if you can not see the accounting calculation).
Opportunity cost is the actual cost to do any activity, because it makes the opportunity which the cost that is hidden. (and can't see if computational account).