Is opportunity cost the rate of return available to an investor for a given level of risk?

No!

Opportunity cost is the cost (sacrifice) forgone (or lost) as a consequence of choosing one option over an alternative that may be equally desired. Thus, opportunity cost is the cost of making one choice rather than another. Every action has an opportunity cost! And it is not restricted to only monetary or financial matters: when a person chooses one leisure activity (option 'A') rather than another that was equally desirable (option 'B'), they 'lose' the pleasure etc that would have been enjoyed had they chosen option 'B'. This 'loss' would be the 'opportunity cost' of making choice 'A'. The exercise of choice invariable restricts the chooser from making certain other choices. There are many mutually exclusive choices in life. A person who chooses to marry does so at the 'cost' of the advantages and freeness of singleness. In economics, 'opportunity cost' is a very important concept, because it forces the decision maker to evaluate, not only the benefits of pursuing a certain course, (e.g. course 'A'), but also to evaluate the direct loss of other opportunities that could be taken if course 'A' is not taken. For example, an investor may decide to put all his money into financing a particular project. But, by using the 'opportunity cost' concept, he also is well aware that he does so at the 'opportunity cost' i.e. the cost of lost opportunity, the opportunity to get, perhaps bigger or more secure returns elsewhere.