Suppose that Sandy has a bakery. When she is running the bakery, her opportunity costs consist of all the things (costs) she has to give up to do so. If she did not run the bakery, but sold lemonade instead, she would earn 10 dollars per hour. Therefore, her opportunity cost of running the bakery includes the $10, since she can not get if she is spending her time working for the bakery.
Work Step by Step
However, an accountant would not count the $10 as a cost, because accountants only keep track of actual money inflows and outflows, which is not the case with the 10 dollars mentioned above, because that is an implicit cost.