Marian Hossa and Ben Smith also scored for the defending Stanley Cup champions, who have won three in a row and can wrap up the first-round series at home Sunday. Four of the first five games in the series have gone to overtime.
All three of the Blues' home games went to overtime and St. Louis won the first two. Toews got behind the St. Louis defense on the winner and threw a couple of fakes at Ryan Miller before slipping the puck past the goalie.
Before Toews scored, the top offensive threats in the series had been neutralized in Game 5. Patrick Kane and Toews combined for one assist in regulation and the Blues' Vladimir Tarasenko, who entered with an NHL-leading four goals in the playoffs, was held to two shots.
The first four goals came from players scoring for the first time in the series.
Pietrangelo entered averaging more than 31 minutes in the series, most on either team, but has been a reluctant shooter with five shots in the first four games. His second career playoff goal came on a 2-on-1 break with Jaden Schwartz with the teams at 4-on-4 to tie it at 2 early in the third.
David Backes returned for the Blues while Blackhawks defenseman Brent Seabrook finished a three-game suspension from the illegal hit that knocked the St. Louis captain woozy in Game 2. But the Blues didn't make the most of that emotional lift early, getting their first scoring chance at about the 12-minute mark, totaling one shot on two early power plays and trailing 1-0 after the first.
The Blackhawks started the scoring for the third straight game, capitalizing when Blues defenseman Roman Polak got caught pinching at the blue line. Bryan Bickell led a 2-on-1 break and Hossa tapped his own rebound into a wide open net at 16:11 of the first period.
On his knees, Oshie lifted a backhander over Corey Crawford's stick to tie it at 11:04 of the second period. Six minutes later, Smith tapped in a backhander after a Blues defender stopped Sheldon Brookbank's slap shot from the point.
NOTES: All three games in St. Louis have attracted standing room crowds, with 19,796 showing up on a busy sports night downtown with the defending National League champion Cardinals playing the Pirates just a few blocks east. ... Oshie totaled three shots and was minus-3 his first three games back from an illegal check to the head late in the regular season from Minnesota's Brian Rupp, who was suspended four games. Oshie said Thursday he might have returned a game too soon. ... Hossa entered second in the playoffs with 21 shots but was held to one assist the first four games.