The opportunity cost is what you give up when you make a decision. It includes all the alternatives you give up when deciding to invest in a particular asset.
As an example, when buying a house, the opportunity cost would be the return that the same capital could have given if it had been invested in the stock market.
On a day-to-day basis, the opportunity cost would be what you give up when you decide to do something. For example, if you decide to go to the movies, your opportunity cost could be staying at home lying on the couch, meeting friends or any other alternative plan.