Increasingly hard rain and saturated stage conditions meant those further down the running order - including all the regular lead combatants - struggled with standing water and aquaplaning.
Sixth and seventh on the road, Ostberg and Ott Tanak enjoyed slightly better weather on SS2 in particular, where they surged to the front of the field.
M-Sport Ford driver Tanak, who overshot and went off the road on the first stage, held the lead at that point, albeit only by a tenth of a second.
Ostberg then edged ahead on stage three in his Adapta Ford, surviving a spin to complete the loop with a 2.7s advantage over Tanak. Given that Ostberg had run the whole morning on unsuitable hard tyres, it was a remarkable performance.
Citroen's Mikko Hirvonen is the best of the big names in third place. As conditions improved on the 44-kilometre stage three, he was able to trim a little time back from Ostberg and Tanak, but is still 13.1s off the lead.
Hirvonen's team-mate Sebastien Loeb is 6.4s further back in fourth.
Jari-Matti Latvala led his final rally for Ford after SS1, then struggled badly with the deteroriating weather.
He stopped looking at rivals' splits time for SS3 and went slower still, dropping to nearly a minute away from the lead. The Finn is only 14s ahead of compatriot Jarkko Nikara, whose one-off Prodrive Mini run has got off to an excellent start.
Latvala and Nikara's team-mates are among a host of early retirements.
Petter Solberg broke his Ford's suspension when he hit a rock on stage two, with Thierry Neuville's Citroen Junior DS3 and Andreas Mikkelsen's VW Skoda damaging their cars at the same spot.
Dani Sordo was in the top-five hunt in the other Prodrive Mini before going off on SS3.