Visa approval is a multi-step process. Aliens seeking permission to work in the U.S. in O or P classification must first obtain U.S. Citizenship and Immigration Services (USCIS) approval of a petition filed with one of the two USCIS service centers (California or Vermont). With USCIS permission, the alien then applies for a visa at a U.S. Consulate, or Consular Section within a U.S. embassy, abroad, unless no visa is required, as in the case of Canadian citizens. In either case, the alien then appears for inspection at a port of entry (POE) or pre-flight inspection facility (PFI) at an airport abroad. CBP then admits the alien in the status, or classification, that corresponds with the alien's visa (or petition approval). Once in the U.S., the alien might seek an extension of stay or a change of status by filing a petition or application. See our Filing Timeline for an outline of the many steps involved in the visa petition process.

Visas can last for varying periods of time, depending on a variety of factors. Some B visas can last for ten years. Work-related visas, such as Os and Ps, generally expire at the end of the approved classification period. However, the visa might be for a shorter period if, for instance, the alien chooses to pay a lower fee for a visa of shorter duration or if the alien is from a country to whose nationals the U.S. issues visas for shorter periods for reasons of reciprocity. Indeed, nationals of some countries not only receive visas for shorter periods than normal, they can also be limited to as little as a single entry. For instance, O and P visas for Mexican nationals last for no more than six months, but with no limit on the number of entries for which the visa can be used. Those for nationals of the Peoples Republic of China are valid only for a single entry within a three-month period, while those for Brazilian nationals are valid for multiple entries during a three-month period. The "reciprocity schedule" for particular countries changes every so often.

U.S. consulates are not bound by a USCIS grant of a petition for any classification. They may refuse to grant a visa if the alien, in their opinion, is not qualified for the classification sought or is otherwise excludable for reasons of security, prior visa violations, etc.