Tornadoes were reported in Darke, Mercer, Miami and Montgomery counties, according to weather radio spotters.
Ohio Task Force One has mobilized a search and rescue team in Montgomery County.
Local hospitals said they are treating injuries, but exact numbers were not available early Tuesday morning.
Widespread damage has been reported in the Miami Valley. Schools, homes and businesses have been damaged or destroyed. Tens of thousands are without power, emergency shelters have been opened and Montgomery County residents are being asked to conserve water.
Storm surveys will be conducted today to determine where tornadoes touched down and how strong they were.
The Beavercreek mayor has declared a state of emergency and some areas are under a mandatory evacuation.
In Celina, police are asking people to avoid coming into the city because of storm damage. Power has been restored to about 75 percent of customers.
Brookeville schools are closed for the day after the school complex was heavily damaged.
Damage has also been reported in Trotwood, Harrison Twp., Miami County and several other areas.