Other Critical Costs
Associated with Production

In another sense of the word, opportunity costs are also those costs that you incur by forgoing opportunities open to you

For example, you are here getting a college education.  Because of that you are not out there working full time at Grease Monkey.

So, to be here, you are giving up the money you would have been making in your oil changing career (or any other career).  It is an opportunity cost (and it's why lots of people don't finish college).

sunk