Dale Oen got his international breakthrough in 2005, placing seventh in the 100-metre breaststroke during the 2005 World Aquatics Championships. During the European short-course Championships in December the same year, he swam the 100 m breaststroke in 59.05 seconds, placing fourth and setting a new Nordic Record. He became the first Norwegian to swim this distance in less than 1 minute. At the Norwegian Short Course Championships two months later, he bettered that time to 58.81, a world best mark for the year.