Here is a simple way to keep the two separated. Business Opportunities are just that, an opportunity to make a profit. If you are thinking about buying a Franchise for example, you are buying an opportunity to make money. No Franchise comes to you making money unless of course you are buying an already existing Franchise that is making money.
A Business is just that...a profitable endeavor which you are buying the cash flow and profits to achieve some type of personal or business goal. Generally speaking, if it is not making a profit, it should be classified as a Business Opportunity even if it has been around for years. Some exceptions do exist, where for example, you are buying an ongoing business that has a large asset value but is not presently making money.
In either case be aware of what you are buying. It's value is very much effected by these definitions.