After attending Silverdale School in Sheffield, van Oostrum started his basketball career with British basketball league club Sheffield Sharks in 2008. In May 2009, van Oostrum moved to Spanish ACB team Caja Laboral, who loaned him to the reserve team Euskotren Fundación Baskonia, of the fifth tier of the Spanish basketball league. In 2011, after playing in the 2011 Eurobasket tournament, he was loaned to Bàsquet Tarragona 2017, team of the Spanish second tier league LEB Oro.
After winning the LEB relegation playoffs, van Oostrum was called by Caja Laboral to join the roster for the 2012 ACB Playoffs.
In the summer of 2012, Baskonia sent him on loan to the LEB Oro team Cáceres Patrimonio de la Humanidad. In February 2013, he came back to the club of Vitoria after the departure of Carlos Cabezas.
In September 2014, he was loaned to Cibona Zagreb for one season. On October 31, 2014, Cibona terminated contract with him, so he returned to Laboral Kutxa.
On 14 November 2014, Baskonia agreed a new loan, this time to Finnish team Kouvot.
On 22 December 2014, he parted ways with Kouvot after playing 6 games with the club On 7 January 2015 he was loaned to Leeds Force of the British Basketball League and made his debut three days later, scoring 16 points and 6 assists shooting 6-17 from the field on a loss against Plymouth Raiders. Afterwards he was loaned to Peñas Huesca of the LEB Oro.
On 11 August 2015, Van Oostrum was loaned to Arkadikos of the Greek Basket League for the 2015-2016 season.
Having previously represented England at under 16, under 18 and Britain at under 20 levels, van Oostrum made his debut for the British senior team against the Netherlands scoring 8 points. van Oostrum"s recent performances earned him a spot on the British team that will compete in the 2011 Eurobasket tournament in Lithuania. Devon"s performances recently in 2012 have earned him a place in the provisional 21-man squad for the British Olympic basketball team
Devon is the brother of Nigel Van Oostrum.