How are spoken Oirat and Kalmyk related?

Spoken Oirat and Kalmyk are closely related, as both belong to the Western group of the Mongolian languages. Historically, the terms have sometimes been used as synonyms, though they now carry distinct geographical and script-based associations.1

Linguistically, the designation Kalmyk typically refers to the Cyrillic-written language and dialects of the European Mongols, while Oirat refers to the vertical script language and its associated dialects in Asia. Despite these distinctions in writing systems and regional history, spoken Oirat is little differentiated from Kalmyk as a spoken language, with differences primarily limited to vocabulary.1

The two groups share a common origin as Western Mongols who historically lived west of Lake Baikal. While a portion of the population migrated to the Volga River region in the 17th century—becoming the ancestors of the modern Kalmyk—others remained in or returned to areas of China and western Mongolia, where they are identified as Oirat speakers.12