We could explain the opportunity cost shortly,by saying that you had an offer to work part-time,and you know that this money provided will help you in so many ways.
Now this is an opportunity,but what is the opportunity cost?
Maybe its that you will spend less time with your family/friends,or maybe follow your passion and paint,draw or sing.
Got it? It runs on a simple statement,everything you win,you lose another thing in exchange for it.
Another example is while you're reading this article now,you could've done another million things,but you've chose to read this one,and I salute you for taking such choice.
So the opportunity cost is simply the thing you lose in exchange for the thing you'll win.
Thats why you gotta keep in your head this term,because every opportunity has a cost,and you should know it before you decide what you'd do,because otherwise you'll know what it was,but later when you probably wont have the choice of stepping back.
Know what is the opportunity cost,because you have limited resources,good planning makes a great life.