The province's police watchdog is investigating the deaths on Highway 402, where 25 to 30 vehicles — including a police cruiser — were involved in the chain reaction crash.
Two civilians from separate vehicles died of their injuries, said the Special Investigations Unit.
The highway was closed in both directions after the pileup, which happened around 10:15 a.m. Thursday. Authorities said it was expected to remain closed well into the evening.
Among the vehicles involved was a marked Ontario Provincial Police cruiser that was responding to the scene of previous crash in on Highway 402.
"The SIU investigates any incident involving police where there has been death, serious injury or an allegation of sexual assault," said Monica Hudon, spokeswoman for the agency.
"In this case there was death and there was an OPP officer in the vicinity."
The pileup included trucks and cars, and reports from the scene said the initial crash involved a transport truck and a car. No emergency responders were reported hurt.
Environment Canada had warned of poor visibility due to bursts of blowing snow, with winds of 60 kilometres an hour.
In the Chatham area, police urged motorists to stay off the roads after a number of crashes, and closed part of eastbound Highway 401. Roads were also closed in the London area.
In Hamilton, police closed a section of the Lincoln Alexander Parkway after six collisions but later reopened the road. Several collisions in Cobourg, east of Toronto, forced police to close Highway 401 in both directions. Snow squalls were blamed for those crashes as well.
The crashes came on a day when provincial police said they would be increasing traffic enforcement following a spike in fatal collisions on Ontario highways.
Less than a month into 2012, they said, the number of deaths on provincial police-patrolled highways had more than doubled over the same period last year.
That total of 17 deaths since Jan. 1 did not include the two people killed Thursday. Eight people died in vehicle crashes during the same period in 2011.
Provincial officers will be educating motorists on proper winter driving and will target aggressive, distracted and impaired drivers, the force said.
"We will be relentless in our approach," said Chief Supt. Don Bell, commander of the highway safety division.
"We intend to use every available means at our disposal to stem this needless loss of lives."
(The Canadian Press, CKRU, CHOK, CKLW, CKOT)