|The Umayyad Caliphate at the time of Odo|
Gaul was a land mass, not a country: in that space were numerous areas ruled by different men. Odo was at odds with the political entity we normally think of as ruling Gaul at this time: the forces under Charles "the Hammer" Martel, who was the powerful "Mayor of the Palace" of the Merovingians and whose grandson would be known as Charlemagne and unite much of Gaul under his rule.
Martel's claim to fame (or one of them) was preventing the Muslim invasion of Europe, especially at the battle of Tours in 733. But Odo had already made some progress in that area. Odo's territory was just north of what is now Spain, bordering the Caliphate of the Umayyads. On 9 June 721, Odo defeated a Muslim army under Al-Samh ibn Malik al-Khawlani at the Battle of Toulouse. He then married his daughter to a Muslim lord, Uthman ibn Naissa, making an alliance with the area that would become Catalonia. This seemed like a smart move.
Charles Martel didn't really hold with the idea of making friends with Muslims, however. Moreover, his goal was to possess more territory. He invaded Aquitaine in 731, and while Odo was being defeated by Charles, on his other border Odo's ally Uthman ibn Naissa was being attacked by Abdul Rahman Al Ghafiqi, who defeated Uthman and sent Odo's daughter to a harem in Damascus. As Abdul Rahman advanced, Odo engaged him and was defeated. He had no choice but to turn to Charles Martel for assistance, which was offered on the condition that Aquitaine swear fealty to Charles. So Charles wins at the Battle of Tours, and Odo fell into historical obscurity. In 735 or so he abdicated as Duke of Aquitane; we think he went to a monastery.
Odo was not the eldest son of the Duke of Aquitaine, and got the position when his older brother abandoned his rights to it. That brother was named Hubertus; I'll tell you about him next.