Olsen entered the league as a Project-40 player, and was allocated to D.C. United on December 27, 1997, shortly after finishing his junior season at UVA. In 1998 Olsen excelled as a right midfielder with United. During his inaugural season he earned a starting spot and scored 4 goals and made 8 assists. These accomplishments earned him the 1998 MLS Rookie of the Year Award and supported the team as it won the 1998 CONCACAF Champions Cup and 1998 InterAmerican Cup.