Recently, a small fortified dwelling dating from the 10th century was discovered on the north-east side of the town's limits. An archeological excavation is now in progress.
Ostrów received town privileges in 1404 but the economic stagnation caused by fires, wars, and a weak 16th century nobility, led to the town’s officials dropping its town status in 1711. In 1714, one of the nobility of Ostrów, Jan Jerzy Przebendowski intervened at the royal court, for the status to be reinstated. By the power of a Royal Marshall (English: Marshal), Franciszek Bielinski, the town received its status back with greater privileges. Another noble family, the Radziwiłłowie took patronage over the town and looked over its many investments. The care of the town’s owners, work of its people, dedication of its officials and its location, have favored the town’s continuous growth. In Ostrów, a railroad hub was formed and became a vital point of the town’s development. It also helped to establish its prominent status on the local and national scene. During the time of Partition and both World Wars, the town had become an important source for nationalist movements. One of the town’s historic episodes was the so-called Republic of Ostrów (Republika Ostrowska), which was the citizens’ upheaval of 1918. No blood was shed at that upheaval and all political powers were taken over from the Prussian authorities. In between the First and Second WW, Ostrów was one of the fastest growing towns: the number of inhabitants doubled, showy houses were built and modern Railcar Manufacturing (Fabryka Wagon) began.
During World War II, a Nazi labor camp Staatspolizeistelle Litzmannstadt Arbeitserziehungslager Ostrowo operated within the town's limits, where 193 people died. The town was one of the major anti-Nazi conspiracy centers in the Great Poland region. In 1941, after the Gestapo’s crackdown on the headquarters of the Poznań branch of the underground army Union for Armed Struggle-ZWZ, the headquarters were moved to Ostrów. From here the re-structure of the Poznań region of the Union was conducted.