Toews redirected Sharp's pass beyond Ray Emery 2:08 into overtime to finish with a goal and two assists.
"We've beaten some good teams like Detroit this year and it just shows that we can get up for the big games like this," Toews said. "It's a big two points for us. The main thing is now we have to move forward and get better from here."
Chicago, which trailed 3-1 in the second, has won two in a row after losing six of eight.
"We learned a lesson early in December, where in today's NHL those losses can really pile up," Sharp said. "You have to come to play every night and I think it showed tonight. It's a good confidence-booster for our team and hopefully we can build off of it."
Sharp misfired on a second-period penalty shot, then made up for the miss 38 seconds into the third when he tied it at 3 as he beat the Senators goalie with a shot from the slot off a setup pass by Toews, who also assisted on Brent Seabrook's power-play goal in the second.
"It was just a bad shot," said Sharp, who fired wide right on his attempt against Emery 11:07 into the second. "It just flubbed off the end of my stick. Even the goal that I scored in the third period, I don't know what was going on but thank God it went in and I could stop thinking about the penalty shot."
Despite his earlier shooting difficulties, Sharp wasn't shying away from a scoring opportunity on the play that led to Toews' winner.
"I was looking to shoot but the defenseman was standing in the shooting lane so I know Toews goes to the net all the time, I just zipped it to him as hard as I could," Sharp said.
Havlat, who didn't figure in the scoring, left the game in the second period because of a groin injury. He did not return.
Nikolai Khabibulin made 25 saves, including a stop on Jason Spezza midway through the third in which the puck rolled just wide of the right post. He also made a pair of saves on Heatley later in the period to send the game to overtime.
"He made some big saves for them," Spezza said. "We generated a lot at the end and we couldn't put it in."
Heatley scored his second of the game -- his team-leading 22nd -- early in the second. He also scored midway through the first to restore Ottawa's one-goal lead after McAmmond opened the scoring with an unassisted effort 5:48 in.
Emery made his first start since leaving 5 minutes into a 6-0 win in Carolina on Dec. 12. Emery, who backed up Martin Gerber for the last four games, stopped 26 shots.
Ottawa lost its second in a row, including a 3-2 loss in Atlanta on Thursday that ended their six-game winning streak.
"When you have the lead, you don't want to settle for a point," Spezza said. "It feels like a loss. It is a loss. You get a point for it but we want to win the hockey game."
Senators captain Daniel Alfredsson failed to record a point, ending a 10-game streak. Alfredsson had four goals and 11 assists over the course of the points streak.
McAmmond opened the scoring on a 2-on-1 with Alfredsson as he held the puck after coming down the left side and cut across the goalmouth and past defenseman Dustin Byfuglien to beat Khabibulin.
Lapointe tied it at 1 with a slap-shot over Emery's right pad at 7:12.
Heatley made it 2-1 at 10:43 with a tip-in on a point shot by Wade Redden that was originally waved off by referee Marc Joannette. A video review showed that Heatley's stick was well below the level of the crossbar when it made contact with the puck.
Heatley increased the lead to two with his second of the game 2:46 into the second, but Seabrook beat Emery with a slap-shot from the left point at 7:21 to draw Chicago within 3-2.
Sharp's penalty shot was the Blackhawks' first of the season. ... Chicago backup G Patrick Lalime played for Ottawa from 1999 to 2004. He remains the Senators' all-time playoff leader in games played (41), shutouts (5), wins (21), and goals-against-average (1.76). ... Heatley extended his current points streak to seven games. He has eight goals and four assists during the streak. Heatley holds the Senators' record with a 22-game streak that began with his Ottawa debut on Oct. 5, 2005. That streak is one game short of Wayne Gretzky's league record for longest points streak from a debut with a new team, set in 1988 with Los Angeles.
Copyright by STATS LLC and The Associated Press
Arena: Canadian Tire Centre Location: Ottawa, Ontario
Referees: Marc Joannette, Greg Kimmerly Linesmen: Steve Barton, Pierre Champoux
Attendance: 20,171 (109.0% full)
Team Stat Comparison
Power Play Goals
Power Play Opportunities
Shots on Goal
Dean McAmmond (2)
Martin Lapointe (2)
Assists: Adam Burish, Duncan Keith
Dany Heatley (21)
Assists: Wade Redden, Jason Spezza
Dany Heatley (22)
Assists: Shean Donovan, Dean McAmmond
Brent Seabrook (3)
(Power Play) Assists: Dustin Byfuglien, Jonathan Toews
Patrick Sharp (17)
(Power Play) Assist: Jonathan Toews