The article considers the cultural and linguistic dynamics of Oirat ethnic groups in Mongolia, a small part of the population of the country living in compact settlements for a number of centuries. They include such Oirat ethnic groups as the Derbet, Torgut, Zakhchin, Mingat, Bayit, Uryankhay, Elyut, Khotogoyt, Khoshut, Khoyt and Khoton. The Oirat language belongs to the west branch of Mongolian languages (Altaic language family). It is an old-written language, the Oirat writing "Todo bichg" (Clear writing) was worked out in 1648 by the Oirad scholar Zaya Pandita. Language and culture assimilation occurs under the influence of social factors such as cognate linguistic environment, demographic factor of the ethnos in the given society. The article is aimed to find out the evolution of Oirat dialects in Mongolia, to define whether they are able to develop in cognate environment, to specify the role of demographic capacity, communicative capacity of the languages and language vitality.