Is it correct to say opportunity cost is cost of giving up quantity of what you are selling? In this video (https://m.youtube.com/watch?v=KEYYN-a_x6E), the instructor talks about the marginal opportunity cost and supply curve. (Begins talking about it at after 1:50).

He states that the opportunity cost of giving up a quantity of apples (what she is selling) is the opportunity cost.

Is this correct? I thought the opportunity cost is the cost of giving up alternative things. But he plots the points of "giving up an apple' on the marginal opportunity cost curve, even though she is selling apples, so wouldn't that not be the opportunity cost.
It seems the instructor is referring to the opp. cost of giving up the profit from selling x amount of apples. Which would be an opportunity cost. Opportunity cost isn't strictly alternative based, for example: If I'm selling 10 apples today, the opportunity cost of be only selling 9 apples tomorrow is whatever profit I stand to lose from not selling the 10th apple tomorrow.