Octa’s father, a great Saxon leader, brought Octa to Britain during the reign of King Vortigern, Hengist’s brother-in-law. When Hengist’s plan to conquer Britain became known, Vortigern and his successors went to war with the Saxons. Octa succeeded his father as the Saxon leader after Hengist was executed by King Ambrosius.
Ambrosius defeated Octa, forced baptism on him, and banished him to Scotland. When Ambrosius died, however, Octa resumed his war against King Uther. He was captured with his kinsman Eosa at Mount Damon by Uther, and was thrown into prison. In time, Octa and Eosa convinced the prison guards to free them, and the lot fled to Germany, where they raised an army and returned. Geoffrey says that Octa was finally killed at Saint Albans (or Verulam) by Uther’s army.
In Nennius (whose chronicle predates Geoffrey), it is uncertain whether Octa is the Saxon commander in the battles against Arthur, but the possibility is open. Geoffrey attributes this campaign to Octa’s successor, Colgrim.
Arthur's Battles | The Legend of King Arthur