O'Neill, (Ó Néill), Aed Buide (d. 1283), king of Tyrone, son of Domnall Óc O'Neill (d. 1234), king of Tyrone, is first mentioned in the annals as a significant enemy of Brian O'Neill (qv), the disputed high-king of Ireland. In 1259 he brought a large force to rendezvous with Domnall Óc O'Donnell (qv) (Ó Domnaill), who was preparing to invade Tyrone, and together they burned widely throughout that kingdom before attacking the MacMahons (Meic Mathgamna) of Oriel and taking hostages. Aed Buide's enmity seems to have played a large role in Brian's downfall. The following year saw the start of a momentous period for the O'Neills. On 16 May 1260 the English of Ulster with unnamed Irish allies decisively defeated the combined forces of Brian and Aed O'Connor (qv) (Ó Conchobair), at Down, where Brian was killed; his death allowed Aed Buide to assume the title of king of Tyrone, but shortly afterwards he was banished by his brother Niall Cúlánach O'Neill (c.1235–1291). Niall Cúlánach and his main ally, Donnsléibe MacCawell (Mac Cathmail), were heavily defeated by Domnall Óc O'Donnell later that year. In 1261–2 Aed Buide deposed and expelled Niall Cúlánach from Tyrone and took the kingship again.
Even though he had English support and earned a fee as their guardian of the marches, Aed Buide was ambitious to strengthen his position further. In pursuit of power he ruthlessly persecuted Niall Cúlánach and his allies, and in 1263 killed MacCawell before expelling Niall Cúlánach once more. By coercing several of his allies to support him, he assumed the over-lordship of MacMahon of Oriel in 1264 and captured Cúmuige O'Kane (Ó Catháin). He seems still to have been reliant upon English support and formed a close relationship with Sir Henry de Mandeville, then keeper of the crown lands in north Antrim. He also cultivated links with Walter de Burgh (qv), earl of Ulster and lord of Connacht: about 1263–4 de Burgh gave his kinswoman Eleanor de Angulo in marriage to Aed Buide and in 1265 the two campaigned together against the O'Donnells of Donegal. The alliance was confirmed by de Burgh's charter dated 2 October 1269, in which the earl warned Aed Buide not to mistreat Eleanor, which may suggest that he was an abusive husband or that he wanted to be released from the marriage and from de Burgh's patronage. In 1271 Aed Buide and O'Kane submitted before the justiciar, James de Audley (qv), receiving gifts of robes, saddles, and furs.
In 1273 the mayor and people of Carrickfergus wrote to Edward I, complaining that Aed Buide and the O'Kanes had begun a war against them at the instigation of Sir Henry and Sir Robert de Mandeville, who were engaged in a feud with William Fitzwarin, seneschal of Ulster. The damage to the colony in Ulster was extensive: according to the letter, five towns had been burned before Aed Buide and his allies were routed by the seneschal. Aed Buide's defeat prompted Niall Cúlánach to offer his services to the king; he claimed that some on the Dublin council were protecting Aed Buide, and asked the king to allow him to pursue and destroy his weakened brother. His suspicions seem to have been confirmed when, later in the year, Aed Buide was quickly and leniently granted a pardon for his offences. The struggle, however, dragged on until 1276 and culminated in the defeat of the Mandevilles and their allies, though Aed Buide survived and continued as ruler of Tyrone. His greatest victory was won in 1281 at Disertcreaght, 10 miles north of Dungannon, where, with the aid of the English of Ulster and Thomas de Mandeville, he repelled the invading forces of Domnall Óc O'Donnell and killed his former ally. O'Donnell may have attacked Aed Buide at the behest of his son-in-law Domnall O'Neill (qv) (d. 1325), son of Brian O'Neill.
Aed Buide was killed in 1283 by Brian MacMahon (qv) and Gilla Ísa O'Reilly (Ó Raigillig). His descendants were the O'Neills of Clann Aeda Buide but his immediate successor as king of Tyrone was Domnall O'Neill. Domnall's right to the title was disputed by the ageing Niall Cúlánach and in 1286 Richard de Burgh (qv), earl of Ulster and lord of Connacht, led an expedition into Tyrone, deposing Domnall in Niall Cúlánach's favour; Domnall expelled Niall Cúlánach from Tyrone in 1290, but was again himself defeated the following year by de Burgh and his allies the Bissets and Mandevilles, who restored Niall Cúlánach to the kingship. However, once de Burgh had departed, Domnall had his rival killed, an act that enraged de Burgh, who banished him and inaugurated Brian O'Neill (c.1265–1296) as king of Tyrone. Brian Ballach was a son of Aed Buide and he was to reign for four troubled years, during which he faced continual opposition from Domhnall and his family. Even though Brian had strong English support, he was unable to check the growing power of Domnall, and he died in battle against him in 1296.