On 5 July 2018, Van Oostrum signed with New Heroes Den Bosch of the Dutch Basketball League (DBL) and the FIBA Europe Cup. On 3 October, Van Oostrum made his debut with Den Bosch in the FIBA Europe Cup qualifying game against Balkan, scoring 17 points and recording 9 rebounds and 3 assists. in the On 4 October, he played four minutes in his DBL debut in a win over Feyenoord Basketbal. In August 2019, van Oostrum signed a two-months contract with CBet Prienai of the Lithuanian Basketball League (LKL). Club had an option to extend the contract for remaining of the season but decided against it and player was released on 27 October 2019. On 4 December 2019, Van Oostrum returned to the Netherlands to sign with Landstede Hammers. On the Hammers, he joined his older brother Nigel Van Oostrum who has been on the team since 2016. Van Oostrum's international debut was for England in the 2008 Under-16 European Championships. He represented Great Britain at under 20 level, and made his debut for the British senior team in a friendly against Canada. In 2011, he played for Great Britain at the EuroBasket finals in Lithuania and was named British Basketball's Emerging Athlete for the year. He missed out on a place in the Team GB squad for the 2012 Summer Olympics. He also played at the EuroBasket 2013 in Slovenia, and in EuroBasket 2015 qualification. Van Oostrum is the son of Duco van Oostrum, who played basketball at the precursor of Aris Leeuwarden in the Netherlands and since 1995 teaches at Sheffield University. Devon's older brother Nigel van Oostrum plays at Landstede Basketbal since 2016. He holds dual citizenship between the United Kingdom and the Netherlands.