Olney is a census-designated place and an unincorporated area in Montgomery County, Maryland, United States. It is located in the north central part of the county, ten miles north of Washington, D.C. Olney was largely agricultural until the 1960s, when growth of Washington, D.C.'s suburbs led to its conversion into a mostly residential area. It has a total population of 35,820 as of the 2020 United States census.