The pileup is near the Drumbo Road exit ramp, OPP said.
The cause of the crashes has not been determined.
"Outside of people not driving according to road and weather conditions, that would be the early indicators of what happened," said OPP Sgt. Dave Rektor.
"But we'll have to wait until the OPP technical collision investigators arrive on scene and make a more educated determination."
Police initially reported that up to 60 cars were involved in the pileup. That number has since been revised to 20.