Ocupal, is what geographer Alberto Vuletín calls a "paleo-toponym" (ancient place name). In his book on La Pampa province toponyms he mentions it as one of the names given by the natives of that region (Araucanized Tehuelches) to the Salado River.
It is interesting because its name refers to a strange animal.
As there are many Salado Rivers in Argentina, I will specify which one by means of the following map showing the Colorado (in orange) and Desaguadero - Salado - Curacó(in yellow) river drainage basins: 
The river, which changes its name along its course is nearly 1,500 km long (932 mi.), and has its northernmost sources in the province of La Rioja high in the Andes at 5,500 m altitude (18,000 ft.). It flows in a North-South direction parallel to the Andean Cordillera and receives the flow of many rivers which also have their sources in the Andean glaciers such as the Jáchal, Vinchinas, Atuel, Tunuyán and Diamante. Many of these rivers are now used to irrigate the vineyards at the foot of the Andes in the Cuyo region provinces of La Rioja,Mendoza and San Juan.
This has led to a dramatic drop in the flow of the Desaguadero River. Its wetlands (such as Guanacache and Bañados del Atuel have dried up due to this cause). It has also led to friction between the province of La Pampa (downstream) and the provinces of Cuyo region (upstream) about water usage rights.
So, until the early 1900s, the river carried plenty of water and the western region of La Pampa was more humid than it is nowadays.
It is known as Desaguadero along the border between the provinces of San Luis and Mendoza, but after receiving the inflow from the Atuel River in the province of La Pampa (36°16'S) it is known as Salado ("Salty" due to its brackish water) or by the Mapuche language words Chadileuvú or Chadileo which mean the same thing (salty river).
It drains into the salt water lakes of Urre Lauquen and La Salada, and then, changes its name to Curacó (stone water) and continues southward until it flows into the Colorado River at (38°50'S, 64°58′W).
Its basin covers a surface area of 260,000 km2 (one hundred thousand sq. mi.).
According to Vuletín :
Its meaning [Ocupal] has been sought but nobody has found anything concrete about it, though it may seem to refer in a vague manner to some animal that existed in the area and that nowadays has disappeared 
He wonders if it could be the Aguará or maned wolf, or, "the name of the animal of a "horrid howl", mentioned by the native Mariqueo" in the early 1800s.
He states that it is also spelt as: Osocopal, Osopal and locates the place in the Limay Mahuida Department, Lot 19, Fraction A, Section XIX. Which is roughly located at 37°07'S, 67°03'W. At the southern tip of Vutaló Creek and to the east of El Potrol Stream, west of the Chadileuvú River and north of the Limay Mahuida hills.
Regarding Potrol, Luis de la Cruz, who trekked through the area from Antuco Chile to Buenos Aires Argentina in 1806 (when it was still a wild country in the hands of the natives), said that at a lake there lived the ñirrivilu snake-fox. His guide "the Indian Pulemanque told him that the Salado used to be known as Ocupal and that it ran in the Potrol's bed" .
Oop, ocupal, the same beast
The monster of a "horrid howl" is mentioned by de La Cruz, as an "Oop" and it was named after its high-pitched yell (follow the link above Aguar&aacutr; for more details on Oop).
What is the relationshp between Oop and the faintly similar word "Ocupal". A lot! Actually the suffix "al" at the end of Ocupal turns what precedes it into a collective noun, thus "Ocupal" is the collective of "Ocupa", which is very similar to "Oop".
Ocupal would mean "pack of Oop" or "many Oop".
The other ways of spelling it, Osocopal, Osopal seem to imply that the beast was known as Osocop or Osop, both of which are similar to "Oop".
I want to point out that the word "oso" in Spanish means bear, but there are no bears in this part of South America (at least nowadays; see my post on Bears in Patagonia).
 Alberto Vuletín, (1972). La Pampa, Grafías y etimologías toponímicas aborígenes. B. Aires, Eudeba. pp. 148 and 106.
 Image by Knusser; Own work using Digital Chart of the World and GTOPO data. It is licensed under a Generic Creative Commons Attribute / Share-alike 3.0
Patagonian Monsters - Cryptozoology, Myths & legends in Patagonia2011 International Year of Forests Copyright 2009-2011 by Austin Whittall ©