The Ondaine has its source in the woods of Neyranche in the commune of Planfoy[1] at an elevation of 1,020 metres (3,350 ft),[2] where it is called the Ondenon.

Its basin covers 122 square kilometres (47 sq mi)[1] mostly in the department of the Loire, but also in part of Haute-Loire.

It flows into the Loire at the locality called Pertuiset in the commune of Unieux at 430 metres (1,410 ft) above sea level opposite Caloire.

The Ondaine flows through the department of the Loire, passing from upstream to downstream through the communes of Planfoy (source), Saint-Genest-Malifaux, La Ricamarie, Le Chambon-Feugerolles, Firminy, Fraisses, Saint-Paul-en-Cornillon, Unieux (confluence) and Caloire.

It now houses fish species such as brown trout, minnow, gudgeon, sculpin, stone loach, European chub and perch.