The 2014 Olympic team sprint champion crossed almost 26 seconds clear of her nearest rivals to strengthen her lead in the Tour de Ski.
The Swiss venue was also the scene of her first ever distance World Cup win two seasons ago and the Norwegian now leads the Tour by 32.8 seconds ahead of her team-mate Heidi Weng.
Tour de Ski
Oestberg battles hard to retain Tour de Ski leaders bib
The latter was second on Sunday, followed by American Sadie Bjornsen who continued her strong season with her third podium of the campaign.
Stage three of the Tour takes place on Monday with the 10km pursuit event in free technique.
"I knew it was going to be a tough race in these conditions," said Oestberg.
"My goal this season has been to get better each weekend. My shape is very good right now. Tomorrow I will be chased and it is going to be a tough race."
Weng victorious as cross-country season closes in Switzerland