Nannskog played for Swedish Landskrona BoIS from 2001 when the club played in second tier Superettan. He was involved in the promotion of Landskrona BoIS to Swedish top tier league, Allsvenskan. And his success continued during the following year. Already in the first match, Nannskog scored a goal as Landskrona BoIS in a remarkable match defeated their old rivals Helsingborgs IF as Landskrona BoIS won 6–2 in front of an attendance of 12.000 at Landskrona IP. Together with BoIS' other forward, Danijel Milovanović, both of them became a feared top-duo that led their recently promoted club to the top position of Allsvenskan at the break for the World Cup in Japan and South Korea. After the eight first matches, Landskrona BoIS had won five matches, drawn three times and never lost. When forward colleague Milovanović got seriously injured against rivals Malmö FF (an injury from which Milovanović never recovered), this surprising success ended for Nannskog's club, but Nannskog himself continued to score.