First time, he was appointed to the Polish national team by Andrea Anastasi in 2011. He won a bronze medal of the 2011 FIVB World League and the 2011 CEV European Championship. In November 2011 Poland won a silver medal of the 2011 FIVB World Cup and therefore qualified for the 2012 Olympic Games in London. He is a gold medalist of the 2012 FIVB World League in Sofia, Bulgaria. In 2015 he was called up to the team B of the Polish national team led by Andrzej Kowal. Kosok took part in the 1st edition of the 2015 European Games.