Østberg, who took the category victory at Rallye Monte-Carlo, initially trailed local youngster Ole Christian Veiby by 7.0sec after the opening Finnskogen speed test. However, the Norwegian battled hard to claim fastest times throughout all three remaining stages.
After the fan-favourite Torsby Sprint blast, he led Veiby by 8.4sec despite battling with a minor setup issue on his Citroën C3.
"It's been a difficult day for sure but no one could really anticipate what we are seeing this weekend. We have been handling a small issue with brake balance on the car today, so we will make some adjustments overnight and hopefully, tomorrow will be a little bit easier for me," explained Østberg
Pontus Tidemand, who drove a Fiesta World Rally Car here in 2019, was 9.9sec further back in a Skoda Fabia. He led M-Sport Ford Fiesta man Adrien Fourmaux by 30.8sec, the Frenchman lucky to get away with an overshoot at a junction in SS4.
Nikolay Gryazin was fifth in a Hyundai i20, whilst Rhys Yates rounded out the top six in a second M-Sport Fiesta.