Oskaloosa is a city in, and the county seat of, Mahaska County, Iowa. The population was 11,558 in the 2020 U.S. census. In the late nineteenth and early twentieth century, Oskaloosa was a national center of bituminous coal mining. Today, Oskaloosa is home to William Penn University, a private university, and Lake Keomah State Park which is located four miles east of the city.
The town was formally platted in 1844 when William Canfield moved his trading post from the Des Moines River to Oskaloosa. The town was designated by the legislature as the county seat in the same year.
The Des Moines Valley Railroad built north from Eddyville, Iowa through Oskaloosa to Pella, Iowa in 1864. In 1873, this became the Keokuk and Des Moines Railroad, and in 1887, it was leased by the Chicago, Rock Island and Pacific Railroad. The Central Iowa Railway followed, which became the Iowa Central Railway in 1888 and was absorbed by the Minneapolis and St. Louis Railway in 1901. In 1883, the Burlington and Western Railway reached Oskaloosa; this was a narrow gauge line that was widened to Standard Gauge in 1902 and then merged with the Chicago, Burlington and Quincy Railroad.
On January 6, 1882, most of the buildings in the north half of Oskaloosa were severely damaged and most of the plate glass windows in the area were broken by an explosion. Three boys were killed in the explosion. The boys had been seen shooting at the A. L. Spencer gunpowder magazine half a mile north of the town center.
The first bituminous coal mine in the area was opened shortly after 1853 by Robert Seevers, who drove a drift into a four-foot coalbed in an exposed creek bank east of town. Initially, coal was mined entirely for local consumption, but with the arrival of the railroads, coal from the region was shipped widely. In the 1880s, more than one million tons of coal was mined in the county from 38 mines. By 1887, the report of the state mine inspector listed 11 coal mines in or very close to Oskaloosa. By 1895, the coal output of Mahaska County surpassed that of all other Iowa counties, and production had reached more than one million tons per year. In 1911, coal mining was reported to be the primary industry in the region. In 1914, the Carbon Block Coal Company of Centerville produced more than 100,000 tons of coal, ranking among the top 24 coal producers in the state.
Several major coal-mining camps were located in the Oskaloosa area. Muchakinock was approximately five miles south of town, on the banks of the Muchakinock Creek. Lost Creek was a company town and post office with a population of about 500 in 1905, located about 10 miles south of town. On January 24, 1902, there was a mine explosion in the Lost Creek No. 2 mine. This was one of only two major mine disasters in Iowa between 1888 and 1913. A miner setting shots to blast coal from the coal face re-used a hole left over from a previous failed shot, and the result was a coal dust explosion that detonated barrels of gunpowder stored in the mine. Twenty men died on the site and 14 more were badly injured. The explosion sparked a statewide miner's strike. As a result, in April 1903, the legislature enacted a law to regulate blasting in coal mines.
According to the United States Census Bureau, the city has a total area of 7.45 square miles (19.30 km2), of which 7.43 square miles (19.24 km2) is land and 0.02 square miles (0.05 km2) is water.
According to the Köppen Climate Classification system, Oskaloosa has a hot-summer humid continental climate, abbreviated "Dfa" on climate maps.
As of the 2020 census, Oskaloosa had a population of 11,558, with 4,664 households and 2,738 families. The population density was 1,441.0 inhabitants per square mile (556.4/km2). There were 5,181 housing units at an average density of 646.0 per square mile (249.4/km2), of which 10.0% were vacant. The homeowner vacancy rate was 2.2% and the rental vacancy rate was 7.5%.
Of households, 27.9% had children under the age of 18 living in them. Of all households, 42.5% were married-couple households, 19.9% were households with a male householder and no spouse or partner present, and 29.5% were households with a female householder and no spouse or partner present. Non-family households made up 41.3% of all households. About 34.1% of all households were made up of individuals, and 15.0% had someone living alone who was 65 years of age or older.
The median age was 37.3 years. 22.5% of residents were under the age of 18 and 19.4% were 65 years of age or older. The age distribution was 25.4% under 20, 9.2% from 20 to 24, 23.6% from 25 to 44, and 22.5% from 45 to 64. For every 100 females there were 94.5 males, and for every 100 females age 18 and over there were 91.9 males age 18 and over. The city's population was 48.6% male and 51.4% female.
99.3% of residents lived in urban areas, while 0.7% lived in rural areas.
As of the census of 2010, there were 11,463 people, 4,715 households, and 2,842 families residing in the city. The population density was 1,542.8 inhabitants per square mile (595.7/km2). There were 5,144 housing units at an average density of 692.3 per square mile (267.3/km2). The racial makeup of the city was 94% White, 1.5% African American, 0.3% Native American, 1.5% Asian, 0.9% from other races, and 1.8% from two or more races. Hispanic or Latino of any race were 2.4% of the population.
There were 4,715 households, of which 29.7% had children under the age of 18 living with them, 44.0% were married couples living together, 12.2% had a female householder with no husband present, 4.1% had a male householder with no wife present, and 39.7% were non-families. 33.3% of all households were made up of individuals, and 15.1% had someone living alone who was 65 years of age or older. The average household size was 2.30 and the average family size was 2.92.
The median age in the city was 35.8 years. 23.4% of residents were under the age of 18; 13.1% were between the ages of 18 and 24; 23.3% were from 25 to 44; 23.2% were from 45 to 64; and 17.1% were 65 years of age or older. The gender makeup of the city was 48.8% male and 51.2% female.
As of the census of 2000, there were 10,938 people, 4,603 households, and 2,863 families residing in the city. The population density was 1,593.8 inhabitants per square mile (615.4/km2). There were 4,945 housing units at an average density of 720.5 per square mile (278.2/km2). The racial makeup of the city was 96.00% White, 1.04% African American, 0.25% Native American, 1.32% Asian, 0.04% Pacific Islander, 0.41% from other races, and 0.94% from two or more races. Hispanic or Latino of any race were 1.26% of the population.
There were 4,603 households, out of which 29.4% had children under the age of 18 living with them, 49.2% were married couples living together, 10.1% had a female householder with no husband present, and 37.8% were non-families. 32.8% of all households were made up of individuals, and 15.7% had someone living alone who was 65 years of age or older. The average household size was 2.28 and the average family size was 2.89.