His fortune, made with the Volmac Software Group, was estimated by Dutch financial magazine Quote at €1.1 billion at the time of his death.

[3][4] From 1992 to 2011 he staged the annual Melody Amber chess tournament in Monaco, where world-class Grandmasters played rapid and blindfold games.

His other daughter was the eponym to the Crystal Kelly Cup, an invitational tournament for three-cushion carom billiards, which has been held between 1994 and 2011 mostly in Monte Carlo and Nice.

This achievement, however, has been criticized, as at the time of the championship van Oosterom had hired the strong grandmaster Jeroen Piket as his personal secretary.

Earlier, van Oosterom had had two Dutch International Masters on his payroll whose job was to analyse his correspondence games.