"Tomorrow may be even more of a dilemma than today because we're going to get even a little bit more colder," said John Pike, a meteorologist in the Weather Service's office in Norman, Okla.
Five traffic deaths were blamed on icy roads in Oklahoma.
More than 130,000 customers lost power in Missouri, Oklahoma, Illinois and Kansas, utilities reported.
Some communities in Missouri reported ice as thick as three-quarters of an inch, the National Weather Service said.
"The rural roads are pretty rough, the main highways are pretty clear, and the overpasses are slick," said John Christiansen, emergency management director in Missouri's St. Clair County.
Missouri Gov. Matt Blunt declared a state of emergency and activated the National Guard.
Chicago's O'Hare International Airport, one of the nation's busiest, canceled more than 400 flights. The airports in Kansas City, Mo., and St. Louis also canceled several flights.
Places of worship across the region called off services because of the slippery roads. Roads in all but the southeastern corner of Oklahoma were considered slick and hazardous, the state Department of Transportation said.
Chicago officials used the city's emergency phone system to deliver recorded warnings to about 2,700 elderly residents that sidewalks were icy and slippery.
