Since the age of 15, Oostman played at the youth academy of East Side Sparks. In early 2014, he started to be noticed for his talent, and in the summer of 2014 he was signed by Brunanter club FC Drenthe, and put on their reserve team Dinamo Drenthe 69. He performed well in the 2014-15 season, and has attracted the interest of numerous Brunanter and Southern Arc clubs. In the summer of 2016, he was promoted to FC Drenthe's main squad.
Oostman played for the Southern Arc Islands national under-20 football team from late 2014 until 2015, and was a key forward for the team.