Østberg, driving a Citroën C3 R5, ended Friday’s Monza leg in third, but quickly moved up a place when title rival Pontus Tidemand was caught napping in the opening Selvino speed test.
Snowy conditions brought bags of drama, and when Adrien Forumaux punctured a tyre on his Ford Fiesta Rally2 during SS9, Østberg capitalised to claim first.
Two afternoon stages were cancelled on safety grounds and the Norwegian arrived back to service with a 30.9sec advantage over Tidemand. As it stands, Østberg is in prime position to be crowned champion on Sunday.
"It's been very up and down, but a good end to the day for sure. We will see how things go tomorrow, but I am happy to be in this position," he said.
Tidemand was caught out with a bad tyre choice on the opening loop. The Swede opted for a winter tread package, but soon regretted his decision after finding more standing water than snow on the roads. Team-mate Jan Kopecky was 40.5sec further back in third.
Fourmaux’s day went from bad to worse when suspension damage cost him almost 30 minutes. After starting Saturday with the lead, the Frenchman now trails Østberg by 28min 36.5sec
Full coverage from ACI Rally Monza is available on WRC+ All Live here, including every stage broadcast as it happens as well as key interviews, features and expert analysis from the service park.