O'Fallon enjoys four distinct seasons, with none being overly severe. The coldest month is January with an average temperature of 35 degrees F, and the warmest month is July with an average temperature of 89 degrees F. O'Fallon averages 34 inches of rain and 20 inches on snow per year. The driest month is January and the wettest is May.

The City of O’Fallon is located 12 miles west of St. Louis County, bordered by Interstate 70 on O’Fallon’s north side and by Interstate 64 (US 40/61) on the south. Also located within the city limits are the City’s primary north-south thoroughfares: MO State Highway 79, Highway K and Bryan Road.
Other major highways/interstates in the metropolitan area include: Interstate 270, Interstate 44, Interstate 55, Interstate 370 and MO State Hwy. 94

The City of O’Fallon is home to four overthe-road truck terminals: AC Trucking, B&Z Logistics, Heartland Express and S&H Transportation.

Located just 14 miles east of the City of O’Fallon is Lambert International Airport. Passenger and air cargo service is available nonstop to most U.S. cities by the following carriers: Air Canada, Air Choice One, Air Tran, Alaska, American, Cape Air, Continental, Delta, Frontier, Southwest, United, USA 3000 and U.S. Airways. Airports that service private and corporate flights include:
• Spirit of St. Louis Airport, located only minutes away in nearby Chesterfield, offers a 7,003 ft. runway with full instrument landing systems.
• MidAmerica St. Louis Airport serves as the St. Louis region’s second full service commercial airport and is located in St. Clair County, Illinois.
• Smartt Field, owned by St. Charles County, is located near Portage de Sioux, Missouri.
• Creve Coeur Airport is a small general aviation airport located in Maryland Heights, Missouri.
• St. Louis is en route to becoming a cargo hub for direct trade with China. If efforts are successful, this would make St. Louis only the third point of entry for China into the United States, as well as its gateway to the Midwest.