A less probable theory is that Biharea is of Daco-Thracian etymology (bimeaning "two" and harati "take" or "lead"), possibly meaning two possessions of land in the Duchy of Menumorut.

The castle of Byhor, or Bihar (now Biharia in Romania), was the center of the duchy of Menumorut at the time of the Hungarian conquest of the Carpathian Basin in the 890s, according to the Gesta Hungarorum.

[10] For instance, historian György Györffy says that Menumorut's name preserved the memory of the Moravians who dominated parts of the Carpathian Basin in the 9th century.

[7] According to historians György Györffy and Victor Spinei, the presence of Kabars in the region could have given rise to Anonymous' reference to Menumorut's "Khazars".

[11][12] Place names of Slavic origin—for instance, Zomlyn (near modern Darvas in Hungary), Csatár and Szalacs (now Cetariu and Sălacea in Romania)—show that Slav communities lived along the rivers Ér and Berettyó and around Bihar.

[16] Archaeologist Erwin Gáll writes that the cemetery at Bihar may represent a "peripheral centre" of a core region which was located along the upper courses of the river Tisza, because the burial customs were similar in the two territories.

If this scholarly theory is valid, the ancestors of the Székelys of Telegd had lived in Bihar County before they moved to eastern Transylvania.

[23] Modern historians agree that the county was established between 1020 and 1050, most probably by Stephen I, the first king of Hungary, or possibly by his successor, Peter.

[9][32][33] Duke Géza, his brother, Ladislaus, and their cousin, King Solomon of Hungary, joined their forces and chased the marauders as far as Doboka (now Dăbâca in Romania).

[40] In a letter, written on 16 May 1204, the pope ordered Simon, the Catholic Bishop of Várad to visit the "Greek" monasteries and to set up a separate diocese, directly subjected to the Holy See, for them.

[41] Pop also writes that the Orthodox bishopric "in the country of Knez Bela", mentioned in a letter that Pope Innocent wrote to the Archbishop of Kalocsa in 1205, was located north of Oradea.

[53] Judge royal Paul Geregye erected Sólyomkő Castle at Élesd (now Aleșd in Romania); his sons held further 2 newly built fortresses in the 1270s.

[55] James Borsa, one of the semi-independent "oligarchs", was the actual ruler of Bihar, Kraszna, Szabolcs, Szatmár and Szolnok counties in the early 14th century.

[55] After James Borsa's fall in the late 1310s, the noble Czibak, Debreceni and Telegdi families became the wealthiest lay landowners in the county.

[57] One of the earliest references to the presence of Romanians in the county—the place name Olahteluk ("Vlachs' Plot")—was recorded in a non-authentic charter, dated to 1283.

[59] Next a charter of 1326 referred to the Romanian Voivode Neagul who "settled and lived" (considet and commoratur) in Nicholas Telegdi's estate at Káptalanhodos (now Hodiş in Romania).

[60] Historian Ioan Aurel Pop writes that the latter charter proves that Nicholas Telegdi's estate had originally owned by Voivode Neagul.

[62] Following the Hungarian Revolution of 1848, in 1850 Bihar was provisionally partitioned into Ober- and Unter-Bihar (Upper- and Lower-Bihar) as part of the District of Großwardein.

Süd-Bihar was centred on the Landbezirk of Großwardein (Hungarian: Nagyvárad, Romanian: Oradea) and also included the Stuhlbezirke (districts) of Ártánd, Élesd, Margita, Szalonta, Belényes and Tenke.

[65] The Circle on the left bank of the Tisza contained eight counties, including Bihar, with the other seven being Békés, Hajdú, Máramaros, Szabolcs, Szatmár, Szilágy and Ugocsa.