Laguna Oca is an oxbow lake formed by an abandoned meander of the río Paraguay, situated just south of the city of Formosa in Argentina's northeastern Gran Chaco region.

Declared a UNESCO Biosphere Reserve in 2001, its roughly 10,500 hectares make it the first urban biosphere reserve in the world — no point within the reserve is far from the provincial capital.

The reserve encompasses a rich mosaic of wetlands, gallery forests, palm-studded grasslands, and flooded channels shaped by the río Paraguay's seasonal rhythms.

Wildlife is abundant: capybaras, coypus, caimans, Neotropical otters, South American coatis, and howler monkeys share the habitat with notable bird species including the cocoi heron, white-throated ibis, and bare-faced curassow.

Visitors can explore by foot or boat, with guided photo safaris and birdwatching excursions available.

The reserve is reachable from central Formosa via Avenida 9 de Julio heading south, then a dirt road into the protected area, or by a roughly 20-minute boat ride from Tres Marías.

Light clothing and insect repellent are strongly recommended given the subtropical climate.