The article considers the issues of Oirat and Kalmyk lexical specificity in relation to the lexicon of ancient and modern Mongolian languages.While considering phonetic, lexical and derivational dialectisms of the Kalmyk language (Derbet, Torgut, Buzava dialects) referring to the contemporary “Kalmyk-Russian dictionary,” the authors came to the conclusion that the differentiated lexicon of these dialects appeared much later, partly, because of the early Russian borrowings. Lexical study of the Oirat dialects of Mongolia revealed a significant number of non-trivial and irregular phonetic correspondences of the Oirat words and the words of other Mongolian languages. It also revealed a number of archaisms preserved both in Oirat dialects and in the Kalmyk language. The specificity of the Oirat and Kalmyk lexicon in relation to that of the other Mongolian languages, presented in the article for the first time, creates the basis for future comparative and historical research in Oirat Studies.