The Oyirad (also spelled Oirat) is an alliance of the western Mongols.

Also called "Eleuth" or "Olood" in different documentations, the Oyirad was originally a Mongol tribal group inhabiting on the Yenisei River. In 1208 it submitted to Genghis Khan rising from Eastern Mongolia. The royal family had marital relations with the imperial family. However it took sides with Arigh Buga against Khubilai. The latter's victory brought hardships for it during the Yuan Dynasty.

After the retreat from China, the Mongols split into two groups, Mongghul and Oyirad, and conflicted with each other. They were conventionally called Döchin (forty) Mongghul and Dörben (four) Oyirad. The four subgroups forming Oyirad are unclear, but considered as (original) Oyirad, Kereyid, Naiman and Barghud. Although there were more than four subgroups later, they were still called Dörben.

In 1439 Esen Tayisi succeeded his father Toghon Tayisi, who was the de facto dictator of the Oyirad. Esen reunified Mongolia and subjugated the Jurchen. In 1449 he invaded the Ming Empire and captured the Zhengtong Emperor at Tumu. Since he did not want to rule China by himself, he withdrew from China. He killed the nominal Khan and became the Khan in 1453, although he was not a descendant of Genghis. However Esen was killed by a follower in the next year and his empire immediately fell apart.

The Oyirad were conquered by Altan Khan of the Tümed, a grandson of Dayan Khan, who reunified and reorganized the Mongols. Layikhur Khan of the Khalkha Mongols also subjected them. They managed to break away from Khalkha by killing the third Altan Khan of the Khalkha in 1623.

The leading Oyirad subgroups in 17th and 18th century were Dörböd, Jüün Ghar, Khoshuud and Torghuud. They formed a loose confederation. In the late 17th century Jüün Ghar took the initiative away from Dörböd.

Torghuud, led by the chief Khoo Örlög, moved to Kalmykia in 1628 because of an internal trouble Khoshuud originally made. They were called Kalmak or Kalmyk by the Turks and Russians. Allied with Russia, Torghuud flourished there during the reign of Ayuki Khan, who was given the title of Khan by the 6th Dalai Lama.

The Oyirad became Tibetan Buddhists in 1615. It was not long before they got involved in the conflict between the Geluk and Karma schools. As requested by the Geluk school, Güüshi Khan of the Khoshuud defeated Choghtu Khong Tayiji in 1637, who supported the Karma school, and conquered Qinghai, followed by unification of Tibet in 1641. Qinghai became home to Khoshuud. They kept a friendly relationshop with the Manchu Empire. In 1717 Jüün Ghar invaded Tibet and killed Lha-bzang Khan, a great-grandson of Güüshi and the fourth khan of Tibet. In 1723 Lobzangdanjin, a grandson of Güüshi, stood up against Qing, but was crushed in the next year. It followed that Qinghai fell under the domination of Qing and was introduced koshuun (banner) administrative unit like Mongolia.

Under Galdan Khan the Oirats became the chief rival of the Manchu Qing Empire and was vanquished by Emperor Kangxi.

A nomad horde that had previously ventured as far as the North Caucasus, namely the Kalmuks, broke up in the 1700s. Some, including the Turgut branch, returned to Jungaria and Kazakhstan and was accommodated by Qianlong Emperor. The other remain in North Caucasus, maintaining their Gyogyu Sect of Tibetan Buddhism to this day, and currently comprise the dominant nationality of the Kalmyk Tangch republic within the Russian Federation. The incumbent president of Kalmyk Tangch, Ilyumzhinov, a typical Russian plutocrat, is also the head of the FIDE.