Thayer is a city in Oregon County, Missouri, United States. The population was 2,201 at the 2000 census. Its original name was Augusta. Thayer is considered a railroad town, as it was laid out in 1882 to be a division point. Currently, the Burlington Northern Santa Fe railroad travels through town. Thayer is located next to Mammoth Spring, Arkansas. Both towns are interconnected in many ways, but naturally have different governing bodies and school systems.