[2] In 1514, Duchess Anna Radziwiłł, who is commemorated in the town with a monument, established four annual fairs and a weekly market, boosting the development of Ostrów.

[2] In the 19th century, the town saw a significant influx of Jewish settlers as a result of Russian discriminatory policies, and according to the 1897 census, 5,660 inhabitants out of 10,471 were Jews.

[4] During the joint German-Soviet invasion of Poland, which started World War II, Ostrów Mazowiecka was captured by Germany on September 8, 1939,[2] and in mid-September the Einsatzgruppe V entered the town to commit various atrocities against the population.

[5] Already on 19 September the Germans arrested nine Poles, including chairmen of local veterans' organizations Włodzimierz Gadomski and Jan Radbalski.

[8] Major Eugeniusz Mieszkowski nom de guerre Ostry unified the resistance organizations into a district of the Home Army under the cryptonym "Opocznik" ("wheatear").

[10] On May 1, 1943, the Home Army blew up the headquarters of the German Arbeitsamt in retaliation for round-ups and deportations of the local population for forced labour.

[2] The German occupation ended in 1944, and Ostrów was restored to Poland, although with a Soviet-installed communist regime, which stayed in power until the Fall of Communism in the 1980s.