The article examines the role of religion in the ethnic formation of the Oirats, especially between the 15th and 17th centuries. The period proved crucial for the social and political development of the Oirats. The sources for the article include both well-known and understudied texts, such as: the Mongolian chronicle The Secret History of the Mongols (Mong. Mongolyn nuuts tovchoo, compiled in 1240), Compendium of Chronicles (Pers. Jāmiʿ al-Tawārīkh, compiled around 1310) by the Persian historian and statesman Rashid al-Din (1247–1318), History of Rashid (Pers. Tarikh-i Rashidi, 1546) by Mirza Muhammad Haidar (1499–1551), The Autobiography of the Fifth Dalai Lama (Tib. Za hor gyi ban de ngag dbang blo bzang rgya mtsho’i ‘di snang ‘khrul ba’i rol rtsed rtogs brjod kyi tshul du bkod pa du ku la’i gos bzang) by Ngawang Lobsang Gyatso (1617–1682), the Mongolian chronicles Erdeniin Tobchi (1662) by Sagang Sechen (1604–?) and Altan Tobchi of Guush Luvsandanzan (17th c.), texts on the history of the Oirats by the Kalmyk historians Gabang Sharab (written in 1737) and Batur Ubashi Tyumen (written in 1819), as well as some newly discovered Oirat sources.First, the paper considers the correlated issues, i.e. the main tribes (nations) of the early Oirats as they appeared in corresponding sources. According to Rashid-al-Din, the Oirats belonged to the group of peoples, who identified themselves as Mongols, though they had not recognized themselves as such in the earlier times. According to Erdeniyn Tobchi, the main Oirats peoples were the Ogelet, Bagatud, Khoit and Kergud, who composed the early confederation of the Oirats, and according to other chronicles, the first three of them had existed in 15th century. Special attention is paid to the problem of the so-called ‘westward exodus’ of the Elet Oirats. The article concludes that was a true event which happened in the mid-to-late 15th century. The main reason was the religious ardor of the famous Oirat ruler Esen Tayiji’s (an active proponent of Buddhism) two grandsons ― Ibrahim and Ilyas — who had been brought up as convinced Muslims. The paper hypothesizes that being leaders of the Elets, the brothers moved along with a significant part of this Oirat people to Moghulistan where probably most of them were put to death by Alachi Khan (Ahmad Khan). This could happen around 1485–1504. The tragic event led to the important transformative changes among the Oirats, namely: a new ‘middle’ confederation was created by the Oirats with Buddhism as the crucially important faith for the very survival of the nation. Besides, over time the remaining Esen’s descendants formed two new peoples ― Dorbets and Dzungars. The echo of the sudden exodus of the Elet Oirats was still evident in the mid-17th century, as The Autobiography of the Fifth Dala-lama mentions some significant damage among six main Oirat peoples. The paper concludes that religion traditionally played an outstanding role in the social and political history of the Oirats.