When you spend $300 on something, the price is $300, but the cost is $300 plus whatever that money would have become. At 7% over twenty years, that is about $1,160. That is the opportunity cost, and it is invisible because it never appears on a statement.
It runs in both directions, and this half gets told less often. Deferring every expense has an opportunity cost too: the years in which you can travel with small children or look after your parents cannot be reinvested. A whole life of optimized decisions is an enormous opportunity cost, paid in the one currency you cannot get back.
Its practical value is not to make you spend less but to make the trade visible. A $300 purchase chosen in full knowledge that it costs $1,160 of future money is a good decision, if you made it with that information in front of you.