In XVII century, the first known ancestor of Generation XI, Pawel Ossowski with his wife Zofia Orlowska, purchased in 1638 the estate of Neloyevskoye of Brest Province, off the Niepokojczyckis, Grigory and Ivan. Pawel had one son, Yakub Ossowski. Besides his father's estate, Yakub also owned the folwark of Yagolki Kislye in Brest Province, which he had bought in 1716 from the Lesecki. With his wife Katerina Baratynska, he had three sons (Simon, Andrzej and Mateusz). It is not known what had happened of two sons of Yakub (Simon and Andrzej). In the genealogical book of Ossowski, they are present on the tree, but no further information is given about them. In the Armorial of the Belarusian Gentry, they are not mentioned at all as descendants of Yakub.
Mateusz Ossowski was born in 1715, most likely in the patrimonial estate of Neloyevskoye. Continuing the tradition set by his father and grandfather, he went on to purchase new lands. In 1754, he bought from Grabowski the folwark of Shylin in Brest Voivodeship; three years later, he became the mortgage owner of the Ovzichi folwark in Brest District from the Ludwik Orzeszko, the Podstoli of 1 Pinsk; in 1760, he bought the Litevsk folwark from Anton Orzeszko, the Marshal of Pinsk. In 1766, by privilege of King Stanislav Augustus, he was granted the title of the Wenden Game-keeper. With his wife Theresia Zdzitowiecka, he had two sons, Ivan and Antoni, and a daughter Anna. Another Mateusz's son Augustin is listed in the family tree, but we know nothing about his fate. Most likely, he died as an infant, so he was placed onto the Ossowski genealogy tree, but was not reflected in the pedigree of the confirmed nobility.