O'Fallon's city seal was designed in 1974 by Michael Donovan, an OTHS freshman, during a contest held by the O'Fallon Junior Woman's Club to create a city flag. In May 1975, the design was finalized by Louis Bradshaw and was then adopted by the city as its official city flag. According to the 2004 book published by the O'Fallon Historical Society, "O'Fallon Sesquicentennial History": ""The design is yellow gold on a blue background—the colors of the high school. The pick and shovel represent the coal mines that were so important to O'Fallon's economy for over 100 years. The locomotive represents the railroad to which O'Fallon owes its existence. The planes represent Scott Air Force Base where many in O'Fallon find employment—both military and civil service.""