Out of the 19 Mazovian gords whose names were recorded in the so-called Mogilno Falsification from 1065 (Fig. 1), the location of only one of them – Osielsk (Oselzch) – remained unknown. Among the many possible locations identified with the name, the hypothesis formulated by Irena Gieysztorowa and Tadeusz Lalik, who advocated for Kozielsk, a small village situated near the border of Kuczbork, had the most supporters. However, locating a settlement which could be a remnant of the gord of Osielsk in that area proved impossible for fifty years. It was only discovered in 2017 by Tadeusz Manista, an amateur researcher and a member of the Żuromin Historical Group, and then verified by the employees of the National Museum of Archaeology and the Author. A large ring-shaped fortified settlement, with a diameter of over 100 m, surrounded by a rampart consisting of an embankment and two ditches on both sides, is located north of Kuczbork and Kozielsk (Fig. 2–4). The pottery sherds obtained from its surface (Fig. 6) should be attributed to the Early Middle Ages (second half of the 12th – the beginning of the 13th century), but the final determination of the time of construction and functioning of the gord may take place only after archaeological excavations are carried out.