He was sent with diplomatic missions to the Pope in Rome in 1633. He negotiated with Brandenburgians of Duchy of Prussia in 1635 and led another diplomatic mission to Emperor Ferdinand II and his parliament in Regensburg in 1636. As a leader of the pro-Habsburg faction at the royal court, he found an ally in the first wife of Władysław IV Waza, Cecilia Renata of Austria, daughter of Ferdinand II. In 1639 and 1641 he once again negotiated with envoys from the Duchy of Prussia. A Catholic, he opposed Protestants and advocated limiting their rights and privileges. In his pro-Habsburg and anti-Protestant stance he was allied with Chancellor Albrycht Stanisław Radziwiłł and Queen Cecilia Renata. In 1643 he was appointed Chancellor of the Crown. A close royal adviser, he often supported king Władysław IV Waza from the House of Vasa, arguing for increasing monarch power, although he was known for limiting and withdrawing his support if he knew it was impossible to win. Nonetheless he was among the few who supported Władysław's plans in the late 1640s to wage an offensive war on the Ottoman Empire. He had few friends among the lesser szlachta, whom he mostly disliked and treated the Sejm and regional sejmiks as a 'necessary evil', although he rarely broke any laws. From 1644 he switched his alignment from pro-Habsburg to pro-French and supported the second marriage of King Władysław with Ludwika Maria Gonzaga. During his life he became the enemy of Adam Kazanowski and Jeremi Wiśniowiecki, whose power diminished with the marriage between Władysław and Cecylia in 1637. Rival of bishop and chancellorPiotr Gembicki, whom he eventually forced to retire from politics in 1642, receiving his position of Great Crown Chancellor. He was not the wealthiest of magnates, his possessions were very small compared to those of Radziwiłłs or Wiśniowieckis, but almost none of them were mortgaged or loaned. In 1635 he funded the expansion of his family castle in Ossolin. Between 1639–1642 he funded the palace in the capital city ofWarsaw. After the death of Władysław IV in 1648 he supported the election of his half brotherJohn Casimir and was instrumental in his election. Ossoliński also was in favour of treaties with the Cossacks, he took part in the negotiations and was an important contributor to the Treaty of Zborów in 1649. He was a well regarded speaker and orator. He was also an author of:
