The 25-year-old former plumber took the title on New Year's Day after winners from 11 countries took part in a special show on Christmas day.
She's So High, the father of two's first release after Pop Idol, was the biggest selling Norwegian single of all time in his home country.
US winner Kelly Clarkson came second in the contest, and Will Young was fifth.
Ant and Dec hosted the show, which also featured Idols from countries including South Africa, Belgium, Canada, and Poland.
Nilsen performed the U2 song Beautiful Day, and told the other 10 competitors: "I love you guys."
The show's creator, Simon Fuller, said he saw Pop Idol as a "global concept" from the start.
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