What is opportunity cost? and should it be considered as relevant cost or not when calculating the cash flows and NPV for a project?

In microeconomic theory, the opportunity cost of a choice is the value of the best alternative forgone, in a situation in which a choice needs to be made between several mutually exclusive alternatives given limited resources.
Assuming the best choice is made, it is the "cost" incurred by not enjoying the benefit that would be had by taking the second best choice available.[1] The New Oxford American Dictionary defines it as "the loss of potential gain from other alternatives when one alternative is chosen".
Opportunity cost is a key concept in economics, and has been described as expressing "the basic relationship between scarcity and choice".[2] The notion of opportunity cost plays a crucial part in ensuring that scarce resources are used efficiently.[3] Thus, opportunity costs are not restricted to monetary or financial costs: the real cost of output forgone, lost time, pleasure or any other benefit that provides utility should also be considered opportunity costs We can say " Opportunity cost refers to the value lost when a choice is made between two mutually exclusive options " Opportunity cost rate is used as an interest rate (discounting factor) to calculate present value of future cash flow.
Where Opportunity cost rate is rate of return that investor could earned on an alternative investment of similar risk.

opportunity cost is the benefits forgone from sacrificing the next best alternative regarding the inclusion of the opportunity cost in the NPV and project cash flow calculation it should not be included , but it should be considered in other decisions related to consuming a scarce resource

Opportunity cost is the cost of not doing some project or cash inflows that will not be earned from the project because of using of the resource for some other use. In other words opportunity cost is the income from alternative use of a resource, which if not used in the project being analyzed, could be used somewhere else for generating the inflows.

While deciding whether to take up or not to take up the project, as it shows the current return from the resource, shall be considered for decision making while calculating the cash flows and NPV for a project.

Oportunity cost: is the forgone benefit by not taking the best alternative, and we must consider this cost as a relevant cost when we calculate NPV and we dont consider it in calculating cash flow

Is the contribution to the operating income that is forgone ضائع for not using the limited resources in the next best alternative use.
This cost is relevant cost when we calculate NPV and we dont consider it in calculating cash flow

The cost of an alternative that must be forgone in order to pursue a certain action.
Put another way, the benefits you could have received by taking an alternative action.
We calculate the NPV for each alternative and take the best alternative generate more positive cash flow