Oglesby is a city in LaSalle County, Illinois, United States. The population was 3,712 at the 2020 census, down from 3,791 at the 2010 census. It is part of the Ottawa Micropolitan Statistical Area. Oglesby was a center for mining coal, limestone, and silica, located near the confluence of the Illinois River and the north-flowing Vermilion River. Oglesby grew from an amalgamation of several mining villages, such as Kenosha, Portland, and Black Hollow. It was originally called Portland, due to the cement mined and manufactured in the area that was similar to Portland Cement from England. It was renamed in 1913 after the coal company and in honor of Richard J. Oglesby, a former U.S. Senator and three-time Governor of Illinois.