The city is named after the river Oulujoki, which originates in the lake Oulujärvi. There have been a number of other theories for the origin of the name Oulu. One possible source for the name Oulu is a word in the Sami language meaning 'flood water', but there are other suggestions. At minimum, the structure of the word requires that, if originally given by speakers of a Uralic language, the name must be a derivative. In all likelihood, it also predates Finnish settlement and is thus a loanword from one of the now-extinct Saami languages once spoken in the area.
The most probable theory is that the name derives from the Finnish dialectal word oulu, meaning "floodwater", which is related to e.g. Southern Sami åulo, meaning "melted snow", åulot meaning "thaw" (of unknown ultimate origin). Two other word families have also been speculated to be related. The first is seen in the Northern Savo dialectal word uula and its Sami counterpart oalli, both meaning "river channel". The second is the Uralic root reconstructed as *uwa, meaning "river bed" (reflected as vuo in modern Finnish, also in derivatives such as vuolas "heavy-flowing"). To either of these roots, some Sami variety would have to be assumed having added further derivational suffixes.