The O'Cahan (Irish: Ó Catháin 'descendants of Cahan') were a powerful sept of the Northern Uí Néill's Cenél nEógain in medieval Ireland.

They held the lordship of Keenaght and Coleraine until the seventeenth century, to which it was commonly referred to as "O'Cahan's country".

A heavily restored effigy at Dungiven Priory is sometimes associated with Cumee, although it appears to date to the last quarter of the fifteenth century, and seems to be that of a later member of the sept. Dunseverick Castle also formed part of the O'Cahan possessions until it was destroyed by Scottish troops under the command of Robert Monro during the Irish Rebellion of 1641.

The majority of Ó Catháin chiefs fled Ulster in the Flight of the Earls in 1607, and under the terms of Surrender and regrant they forfeited their lands to the English crown.

After the Flight of the Earls in 1607, Donnell Ballagh O'Cahan, Chief of the Ó Catháin (and at one time knighted by the English Crown), was captured and sent to the Tower of London, where he died in 1626.