Odo I, Count of Vermandois

Odo I or Eudes I, called the Insane (died after 1085), was Count of Vermandois and Valois from 1080 to 1085 and ruler of Saint-Simon from 1085. The last Carolingian male (from its branch Herbertines), he was the only son of Herbert IV of Vermandois and Adele of Valois, daughter of Raoul III of Valois and Adele of Bar-sur-Aube.

From 1080 he inherited Vermandois from his father and Valois from his mother, heir of Raoul III, although he was disinherited by Herbert IV. He was probably mentally insane. In 1085 the council of barons took the power away from him and gave it to his sister Adelaide, Countess of Vermandois, married to Hugh, son of Henry I of France. Odo I was given Saint-Simon because of his marriage with Hedwig, the daughter of knight Viromandensis. There is no known data about him after 1085 due to lack of historical evidence.

In the 17th century, Claude Rouvroy (de Saint-Simon), favourite of Louis XIII of France, presented a false family tree, stating that he was heir of Odo I. By this he became the ruler of Saint-Simon and added it to his surname.