He was born on 25 September 1743 in the Zblewo parish near Starogard Gdański. In 1759, he joined the Jesuit Order. He studied philosophy in Kalisz and Poznań, and theology in Poznań. After the dissolution of his order, he became the teacher of Antoni Protazy (Prot) Potocki. He travelled abroad with him, and subsequently inspired the business projects of his student who established, among other things, drapery and hat factories at his Machnówka and Łysoboki estates. Ossowski also became the plenipotentiary of Prot Potocki’s bank. He was, among other things, involved in establishing its Kherson branch, which was important for the Black Sea trade. After the outbreak of the Russo-Turkish War in 1787, his involvement in business affairs related to the Black Sea region caused him to advocate a close alliance with Russia, mostly for economic reasons. In 1789, Ossowski published his work O pomnożeniu dochodów publicznych, wynalezienie kapitału publicznego ku zasileniu nowych potrzeb Rzeczypospolitej z oszczędzeniem podatków [“On multiplying public revenues and finding public capital to supply the new needs of the Republic while reducing taxes”] At that time, he was involved with the Dutch bank of Peter de Häan. He also maintained his earlier close acquaintance with Hugo Kołłątaj, and as a result he participated in the work conducted in the early 1790s on reforming the political system of the Republic of Poland, with special emphasis on its economy. In 1791, as part of this work he published the brochure O urządzeniu starostw. Rada i projekt do prawa [“On the district system. Advice and draft of the laws”]. He wanted the political constitution to be supplemented by an economic one. The ambitious plans made by Ossowski and his colleagues (e.g. to establish a national bank) were thwarted by the fact that their drawing up was delayed, many influential figures were sceptical of these proposals, and, above all, the Targowica Confederation formed and the war with Russia started. As late as in the spring of 1792, Ossowski arranged a loan from de Häan’s bank for the State Treasury, but when the king joined Targowica, he stopped the loan.