According to another theory, the name comes from the Celtic (Gaelic) òs, òsa, meaning "slow flowing water".

It is 101.2 km (62.9 mi) long, which makes it the 24th longest river in the Czech Republic.

After a survey by hydrologists, the location of the spring was specified according to the most abundant source and the length of the entire river was redefined.

A notable body of water is also the fishpond Veselský rybník with an area of 81 ha (200 acres).

There lives endangered species of bivalve Unio crassus at the lower river section.