Onoulphus was a Scirian; with his brother, Odoacer, he was raised at the court of Attila, King of the Huns. Following the destruction of the Sciri, who had been incited to break their treaty with the Ostrogoths by Hunimund, king of the Swabians, Onoulphus with his father Edeko joined the Swabian side in the Battle of Bolia in the late 460s, where they were again defeated by the Ostrogoths under their king Theodemir.
Onoulphus joined the Roman army in the 470s and rose through its ranks. He found a protector in the general Armatus, who had him appointed first comes and in 477, magister militum per Illyricum, commander in chief of the Balkan army. In that same year, by order of the Emperor Zeno, Onoulphus killed Armatus, despite his having greatly benefited by the latter's protection (sources states that Armatus lent him a huge sum to pay for a banquet).
Onoulphus kept his office until 479, when he fell out of favour. He then found refuge with his brother Odoacer, who by then had become king of Italy. One of the duties he carried out for his brother was a campaign against Fredericus, who had succeeded his father Feletheus as king of the Rugians. Onoulphus found it necessary to evacuate the remaining Romans and resettled them in Italy. The remaining Rugians fled and took refuge with the Ostrogoths; the abandoned province was settled by the Lombards by 493.
He remained loyal to Odoacer during his brother's war for survival against Theodoric the Great, king of the Ostrogoths, standing with him during the siege of Ravenna. After Odoacer's death, Onoulphus was killed by archers while seeking refuge in a church.