On 19 December 1414 Casimir I took formal possession of his Duchy of Oświęcim, along with the towns of Toszek and Gliwice.

However, this doesn't mean that the Duke of Oświęcim maintain a pro-Polish politics; in fact, he began at the same time his contacts to with the Bohemian King Sigismund of Luxembourg, and even spend some time in his court (there are information that Casimir I, for unknown reasons, received from the King a pension for 3,000 pieces of gold, perhaps as a payment for his mediation with the Teutonic Order, which, had good relations with the Duke of Oświęcim).

Another of town who belonged to the Duke of Oświęcim, Gliwice, become in a major Hussite base on Upper Silesia.

The struggles with the hussites in the region lasted until 1433, when Casimir I was able to recover Gliwice (who was replaced as a Hussite base by the nearby Bytom) thanks to the unexpected help of Duke Nicholas V of Karniów (son of Jan II of Iron, who years before ordered the assassination of Casimir I's father).

They had three sons: By 1433, Casimir I married secondly with Margareta (1410 – 5 July 1459), daughter of Jan II of Iron, Duke of Racibórz.