St. Charles County Sheriff's Department 101 Crossings Industrial Ct, O'Fallon, MO63366 The St. Charles County Sheriff's Department, located at 101 Sheriff Dierker Court in O'Fallon, serves nearly 330,000…More county residents. The department was formerly located in downtown St. Charles until 2005 when its O'Fallon Sheriff's Headquarters was built. The facility contains administrative services, criminal investigations, patrol division, special operations, community education, civil process and the crime lab. It also deals with Metro Air Support, bomb disposal, SWAT, motorcycle, K-9 and Honor Guard. The department works with community senior groups, neighborhood watch programs and DARE and also provides six school resource officers to area schools.