The Oust is a river

River

A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

 in Brittany

Brittany

Brittany is a cultural and administrative region in the north-west of France. Previously a kingdom and then a duchy, Brittany was united to the Kingdom of France in 1532 as a province. Brittany has also been referred to as Less, Lesser or Little Britain...

, France

France

The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

, right tributary of the Vilaine

Vilaine

The Vilaine is a river in Brittany, in the west of France. The river's source is in the Mayenne département , and flows out in the Atlantic Ocean at Pénestin in the Morbihan département . It is 218 km long...

. Its source is in the hills between Corlay

Corlay

Corlay is a commune in the Côtes-d'Armor department of Brittany in north-western France.-Population:Inhabitants of Corlay are called in French corlaisiens.-External links:*...

 and Quintin

Quintin

Quintin is a commune in the Côtes-d'Armor department of Brittany in northwestern France.-Population:Inhabitants of Quintin are called quintinais.-External links: *...

. It flows generally southeast, through the following départements and towns:

• Côtes-d'Armor
  Côtes-d'Armor
  Côtes-d'Armor is a department in the north of Brittany, in northwestern France.-History:Côtes-du-Nord was one of the original 83 departments created during the French Revolution on 4 March 1790. It was created from part of the former province of Brittany. Its name was changed in 1990 to...
  
  : Uzel
  Uzel
  Uzel is a commune in the Côtes-d'Armor department in Brittany in northwestern France. It is about west-northwest of Rennes and north-west-north of Loudéac....
  
  
• Morbihan
  Morbihan
  Morbihan is a department in Brittany, situated in the northwest of France. It is named after the Morbihan , the enclosed sea that is the principal feature of the coastline.-History:...
  
  : Rohan
  Rohan, Morbihan
  Rohan is a commune in the Morbihan department in Brittany in north-western France.It is the home to the house of Rohan, members of which included viscounts, dukes and princes and has had a prominent role in French history.-References:* *...
  
  , Josselin
  Josselin
  Josselin is a commune in the Morbihan department in Brittany in north-western France.-History:St Meriadek is said to have founded a chapel there during the 4th century...
  
  , Malestroit
  Malestroit
  Malestroit is a commune in the Morbihan department of Brittany in north-western France.-Tourism:The canal is a delightful place to spend a little time, and you can watch the barges as they moor up near the centre of Malestroit. It is possible to hire a boat yourself to explore the waterway, and...
  
  
• Ille-et-Vilaine
  Ille-et-Vilaine
  Ille-et-Vilaine is a department of France, located in the region of Brittany in the northwest of the country.- History :Ille-et-Vilaine is one of the original 83 departments created during the French Revolution on March 4, 1790...
  
  : Redon
  Redon, Ille-et-Vilaine
  -Breton language:The municipality launched a linguistic plan through Ya d'ar brezhoneg on 10 October 2008.In 2008, 14.31% of the children attended bilingual schools in primary education.-Economy:* Cargill Texturising Systems - A pectin factory...
  
  

The Oust flows into the river Vilaine

Vilaine

The Vilaine is a river in Brittany, in the west of France. The river's source is in the Mayenne département , and flows out in the Atlantic Ocean at Pénestin in the Morbihan département . It is 218 km long...

 in Redon. The part of the Oust between Rohan and Redon has been made navigable for small ships, and forms part of the Canal de Nantes à Brest

Canal de Nantes à Brest

The Nantes-Brest canal is a French canal which links the two cities of Nantes and Brest through inland Brittany. It was built during the 19th century and its total length is 385 km with 238 locks from coast to coast...

.

The river Arz is among its tributaries.