The Hypatian Codex first mentions Ostroh in 1100, as a fortress of the Volhynian princes. Since the 14th century, it was the seat of the powerful Ostrogski princely family, who developed their town into a great center of learning and commerce. Upon the family's demise in the 17th century, Ostroh passed to the family of Zasławski and then Lubomirski. In the second half of the 14th century, Ostroh, together with the whole of Volhynia, was annexed by the Grand Duchy of Lithuania. Following the Union of Lublin, the town became part of the Crown of the Kingdom of Poland, where it remained until the late 18th century. Ostroh, known in Polish as Ostrog, received Magdeburg rights in 1585. In the 17th century, the town was surrounded by fortifications, with a moat, a rampart and five bastions. In 1609–1753, it was the capital of the Ostrogski family fee tail, founded by VoivodeJanusz Ostrogski, who invited Bernardine monks to Ostrog. Furthermore, the town had a Calvinist academy; among its lecturers was Andrzej Wegierski. During the Khmelnytsky Uprising, the town was burned by the Cossacks, and its Jewish residents were brutally murdered. The Great Maharsha Synagogue, built in 1627, was damaged during this period. Ostrog slowly recovered, and in the second half of the 18th century, it became the seat of a Jesuit college. In 1793, the town was annexed by the Russian Empire, in which it remained until 1918. Railroad lines, built in the 19th century, missed Ostrog, and as a result, the town stagnated. In the interbellum period, Ostrog belonged to County of Zdolbunow, Volhynian Voivodeship of the Second Polish Republic. The town was an important garrison of the Polish Army, and the Border Protection Corps. Here, the KOP Battalion “Ostrog” was stationed, as well as the 19th Volhynian Uhlan Regiment. On July 7, 1920, during the Polish-Soviet War, a battle between a Polish unit under Wincenty Krajowski, and the Bolsheviks of Semyon Budyonny’s 1st Cavalry Army took place. As in 1919 - 1939 Ostrog was located in close proximity to the Polish - Soviet border, special passes were required to enter some districts of the town. Following the 1939 Soviet invasion of Poland, Ostrog was annexed by the Soviet Union, as part of the Ukrainian Soviet Socialist Republic. An unknown number of the town’s residents were forcibly sent to Siberia. The Nazi German occupation of southern Soviet Union resulted in the establishment of the Reichskommissariat Ukraine, with headquarters in Rivne. In the autumn of 1941 several large-scale mass murders took place in Volhynia-Poldolia. On 1 September, 1941 2,500 Jews were shot in Ostrog. Six weeks later the ghetto was disbanded and another 3,000 people were killed. During the first six months of the German-Soviet war 300,000 Jews were killed in the territory of the Ukrainian Soviet Socialist Republic. In total 1,430,000 Ukrainian Jews were murdered in the Holocaust. Landmarks include the Ostroh Castle on the Red Hill, with the Epiphany church and several towers. To the north-west from the castle stand two sixteenth-century towers. The suburb of Mezhirichi contains the Trinity abbey, with a fifteenth-century cathedral and other old structures.
