R. Jacob Joseph of Ostrog (Yeivi; 1738–1791), was the son of a maggid and succeeded his father in 1766, after the latter's death. Jacob Joseph's son testified that he highly esteemed the teachings of the disciples of Israel b. Eliezer Ba'al Shem Tov, the founder of modern Hasidism. Jacob, who lived in poverty and privation, had a deep sense of social morality. His sermons reflected the social upheaval troubling the Jews of Eastern Europe in the 18th century. Jacob Joseph attacked perversions of justice, informing, and bribery, and took to task community leaders and rabbis who gained their appointments through their relations with the authorities instead of through Torah learning. He taught that the poor are closest to God and worthy of attaining the mystic knowledge of the Torah, but the rich are distant from Him. There exists a kind of social-spiritual division in the world in which the learned profess the unity of God above and ensure the provision of abundance below, although personally gaining only a scanty livelihood; in contrast, the ignorant collect the abundance that pours down from Heaven. Despite this distinction, Jacob insisted on the principle of internal Jewish unity, since all the Jewish people are linked with each other. The zaddik's devotion to the Almighty draws all his brethren after him to holiness.

Ostrog records a page in Jewish history when its Jewish population was miraculously saved during the Haidamack raids in the middle of the 18th century, with the help of their Tatar neighbors. They also emerged unscathed when Russian troops in 1792 attacked the synagogue of Ostrog, believing it to be a fortress, in the fighting that preceded the second partition of Poland. In commemoration of their deliverance the Ostrog Jews instituted a "Purim of Ostrog," and the Megillat Tammuz was read in the synagogue on the 7th of Tammuz. Ostrog was one of the most important centers of Jewish religious learning in Poland, its name being interpreted in Hebrew as Os Torah ("the letter of the Law"). Some of Poland's most eminent scholars served as rabbis and principals of the Ostrog yeshivah, which was already in existence by the beginning of the 16th century.