Opportunity theory is simply the idea that people look for opportunity, whether through legitimate means or otherwise, to gain what they desire. It is a crucial element of the theory of deviance.
The origination of opportunity theory lies in the way that society functions. In a society, there is the concept of "norms" and the concept of "deviance." The "norms" of a society are a system of shared values that determine how a society perceives those within it. These norms include values that dictate a standard acceptable lifestyle. When a standard lifestyle established by a society becomes unachievable, people become deviant in attempts to achieve that standard in order to continue to be perceived as within the realm of norm. Opportunity theory is complex in some ways, because there is not only the deviance of deciding to go outside of cultural norms to attain those things valued by society, but the deviance of then looking for opportunities to attain them in the fastest and most reasonable way possible. Those who commit crime as the result of their deviance also look for opportunities that make the crime efficient. A criminal wanting to rob a store, for instance, might look first for one with minimum security and easy accessibility.