R. Jacob Meshullam Ornstein (1775–1839), Galician rabbi and halakhist, son of R. Mordecai Ze'ev b. Moses Ornstein. R. Ornstein, as a young man, married the daughter of R. Ẓevi Hirsch Wahl of Jaroslaw, who contributed greatly toward his material needs. After R. Wahl's death R. Ornstein was proposed as his successor, but because of the violent conflict that the suggestion aroused, he refused to accept the appointment. In 1801 he moved to Zolkiew, where he was appointed rabbi of the town and district. In 1805 he was appointed rabbi of Lemberg (Lvov) and remained there until his death. During his lifetime the Haskalah movement began to spread in Galicia. On the other hand, the ḥasidic movement also gained strength as a result of the establishment of new ḥasidic centers. Although R. Ornstein, who found himself at the center of these two opposing trends, did not incline to Ḥasidism and was regarded as a Mitnagged, he was at the same time opposed to the Haskalah movement and conducted a resolute campaign against it. He was supported in this struggle by his only son, Mordecai Ze'ev, an extremist who was regarded as the driving force in the war against the maskilim. R. Ornstein distrusted the circle of maskilim that was formed in Lemberg around Solomon Judah Rapoport which included N. Krochmal, I. Erter, F. Mieses, and M. Letteris. As a result of the mounting tension between the two sides caused by Rapoport's sharp criticism of R. Ornstein's Yeshu'ot Ya'akov (see below), a ban of excommunication against Rapoport and the leaders of the maskilim in Lemberg was issued in 1816. It has been assumed that R. Ornstein's son Mordecai Ze'ev was its author but that it had his father's approval. The text of the ban refers to the "sins" of the maskilim in studying German and studying the Bible with Mendelssohn's commentary. The maskilim who ridiculed R. Ornstein by referring to him as "the Great Inquisitor of Galicia" translated the ban into German and complained to the government that it was illegal, since it had been forbidden to issue such bans in Austria from the time of Emperor Joseph ii. As a result R. Ornstein was compelled publicly to rescind the ban. Rapoport and the maskilim reacted to R. Ornstein's persecution with scathing articles and satires.

R. Ornstein was regarded as one of the great halakhists of his era, but his main fame rests on his Yeshu'ot Ya'akov, novellae and talmudic disquisitions on the whole of the Shulḥan Arukh (Zolkiew (1828;1809;1809–10). The four parts of the work, with additions from the author's manuscript and the glosses of his grandson R. Ẓevi Hirsch, were published in Lemberg (1863). The work is divided into a long and a short commentary; in the latter he merely gives explanations of the Shulḥan Arukh, but in the former he summarizes the views and arguments of the posekim while resolving the difficulties of the different novellae by casuistic arguments. R. Ornstein also wrote, under the same title (which he also used for his Bible commentary), responsa on the four parts of the Shulḥan Arukh (Pietrkov, 1906). Among the questioners and respondents mentioned in it are R. Moses Sofer and R. Aryeh Leib Horowitz. R. Ornstein's commentary on the Pentateuch was published in 1907.