Odo of Rennes (Medieval Breton: Eudon Pentevr, Modern Breton: Eozen Penteur, Latin: Eudo, French: Eudes/Éon de Penthièvre) [a] (c. 999–1079), Count of Penthièvre, was the youngest of the three sons of Duke Geoffrey I of Brittany and Hawise of Normandy, daughter of Richard I of Normandy.

Also in 1035, after a dispute between Eudon and Duke Alan III, their uncle Judicaël Bishop of Vannes arbitrated, and Alan III gave Eudon the bishoprics of Saint-Brieuc, Saint-Malo, Tréguier and Dol-de-Bretagne, as well as the counties and baronies of Penthièvre, Goëlo, Avaugour and Lamballe.

In 1054 Geoffrey, Viscount of Mayenne in Maine, affronted by William of Normandy's new fortifications at Ambrières, "appealed to Geoffrey, Count of Anjou" who "called to his assistance Guy-William of Aquitaine and Count Eudo of Brittany"[2] In a charter dated to 1056/1060, Eudon ("Eudo") granted land "in pago Belvacensi" (Beauvais, Picardy) to the Abbey of Angers Saint-Aubin (q.v.

By 1056 Conan gained the upper hand in Brittany, and in 1057 he captured his uncle Eudon and chained him in a prison cell.

In 1064, Eudon's liegeman Rivallon I of Dol invited Duke William to join him against Conan, thus initiating the Breton-Norman War of 1064–1065 in which Normandy, Anjou, Dol de Bretagne and the captive Harold Godwinson combined against Conan II, as depicted in three panels of the Bayeux Tapestry.

Orderic Vitalis stated:[3] "God also gave him [Count Eudon] seven sons, who became remarkable for the singular and changeable events of their lives.

[9] Eudon's descendants formed the junior branch of the Breton ducal family, which gained control of the duchy in 1156 under his great-grandson Conan IV of Brittany.