The precise meaning of og is less clear, but it may either be cognate with the Irish Tír na n-Óg (land of youth) or else derive from the Proto-Indo-European *oku ("soon, fast").

In the 1990s, the Environment Agency, working with partners including the Countryside Council for Wales and farming groups, re-engineered the river to recreate pools and riffles with the intention of allowing the river to re-gain its original form and diversity of wildlife.

Passing Braichmelyn village on its eastern bank, it is joined by the substantial tributary, the Afon Caseg, which drains a large area of the Carneddau mountains.

The course turns slightly more westwards as the river passes through Bethesda and becomes progressively more wooded and more deeply cut providing some white water conditions enjoyed by canoeists.

The widening river is then followed by an abandoned railway line (now the Lon Las Ogwen cycle path) through a woodland valley crossing under the main north Wales coast line and the A55 before discharging to the Menai Strait at Aberogwen, near Bangor.