The Olrog’s Gull is an endemic species to the Atlantic coasts of South America. It breeds only in Argentina between 38° 49´ and 45° 11´ S. There are currently fourteen colonies, 11 of which are distributed in the coastal sectors of Buenos Aires Province between the Bahía Blanca estuary and southern Bahía Anegada while three other are found in the north of Golfo San Jorge, Chubut, more than 700 km south of the Buenos Aires breeding grounds (Yorio et al. 2005, P. Yorio, P. García Borboroglu and P. Petracci, unpubl. data). The Bahía Blanca estuary holds over 70% of the breeding population (Delhey and Petracci 2004). The number of recorded colonies and their location in each year has varied, as Olrog’s Gulls often change breeding locations between years (Yorio et al. 1997, García Borboroglu and Yorio 2007a). In the province of Buenos Aires, it has been recorded breeding in Isla del Puerto, Tres Brazas, Golfada Chica, Canal Ancla, Isla Trinidad (Islote Norte, Islote Bastón, Islote Redondo, Islote Sur), Isla Brightman, Islote Norte de Isla Morro de Indio, Isla Gaviota, Isla Puestos, Isla Gama, Banco Nordeste and Isla Arroyo Jabalí Oeste. In Chubut province, it has been recorded breeding at Islas Laguna (Islote Laguna and Islote Luisoni), Isla Felipe and Islas Vernaci (Isla Sudoeste, Isla Noroeste and Isla Oeste Noroeste).
In winter the species disperses north along the Atlantic and Río de la Plata coasts of Buenos Aires province (Favero 1991, Bo et al. 1995, Chebez and Yorio 2008), and reaches Uruguay (Escalante 1970, Aspiroz 2003) and Brazil (Díaz and Mauricio 1988, Sick 2001). There are accidental inland records in Mar Chiquita, Córdoba province (Yzurieta 1995, Torres and Michelutti, 2005). Although in Uruguay it is mainly a winter visitor, recorded from April up to December (Escalante 1984), some individuals have been sighted in summer (Wetlands International 2006, D. Caballero, pers. comm.). Similarly, birds of different age-classes can be found along the Buenos Aires coast throughout the year (Escalante 1966, Savigny and Favero 2005, Silva et al. 2005, Chébez and Yorio 2008). A detailed list of sighting locations and references can be found in Escalante (1984), Collar et al. (1992) and Chébez and Yorio (2008).