Opportunity Cost is simply cash flow you could have earned. 

If your child keeps their savings in their drawer when they could have put their money into a bank saving account, opportunity cost is simply interest they could have earned.

When a company spends money on wasteful activities, opportunity cost is money they could have earned if they did not have to spend it on wasteful activities. Examples of wasteful activities are:

• reissuing incorrect payroll check
• ordering incorrect item
• recalling product
• redoing