The article describes a Buddhist text in Clear Script relating to the gso-sbyong class and discovered in Uvs Province (Western Mongolia) under the title “Bacaq barixu yosun orošibo” - from the perspective of the tradition of home prayers of the Kalmyks. The Oirat manuscript deals with matsg - the one-day Mahayana Buddhist vow - that could be taken by both laity and clergy. Since the tradition is one of the few exact Mahayana vows to be observed by both laity and clergy, it is quite widespread in Tibet, Mongolia and Kalmykia. The text provides recommendations as to how to self-prepare and take the vow and what one is to keep abstinence from once it has been taken. A person who takes the vow undertakes to observe the Eight Precepts and, thus, abstain from: 1) taking life, 2) taking what is not given, 3) lust and non-virtuous behavior, 4) telling lies, 5) using intoxicating substances, 6) entertaining oneself, using cosmetics and decorative accessories, 7) sitting on high and big thrones (chairs) without corresponding need, 8) eating after noon. The matsg vow could be taken not only from priests but also from lay persons who observed the Five Precepts (upasaka vows, in Kalmyk uvshin samnr). Various aspects of the tradition of fast days have been covered in works by E. P. Bakaeva, G. Yu. Badmaeva, V. K. Shivlyanova but no comparative analysis of matsg vow manuals (manuscripts) of the Western Mongols and Kalmyks has been conducted. According to historical sources, the basis for the tradition among the Oirats was laid by Zaya Pandita Ogtorguin Dalai (1599-1662) in the 17th century. Additionally, the paper attempts to analyze some linguistic peculiarities of the Oirat text which is most probably a translation from Tibetan, namely: the use of the written form of the present tense suffix -mui corresponding to the Tibetan particle ‘o; the use of the verb üyiledkü (‘to do’) as an auxiliary (similar to the Tibetan byed ‘to do’) and syntactic constructions that are rather characteristic of the Tibetan language structure
