On September 17, 1804, Lewis and Clark camped on the west bank of the Missouri River near American Island where Oacoma is now located. During the remainder of the 19th century the area was a stopping off place for explorers, fur traders and steamboat men. The township of Oacoma was laid out in 1891 as the county seat for the newly formed Lyman County; the seat was transferred to Kennebec in 1922. The Chicago, Milwaukee, St. Paul and Pacific Railroad arrived to Oacoma in 1905 and Oacoma was known as a "banking post-village". In the 21st century, it is a rest stop for travelers on Interstate 90.
As of the census of 2010, there were 451 people, 205 households, and 135 families residing in the town. The population density was. There were 236 housing units at an average density of. The racial makeup of the town was 88.9% White, 0.2% African American, 6.4% Native American, 0.4% Asian, 0.7% from other races, and 3.3% from two or more races. Hispanic or Latino of any race were 1.8% of the population. There were 205 households of which 27.8% had children under the age of 18 living with them, 52.7% were married couplesliving together, 8.3% had a female householder with no husband present, 4.9% had a male householder with no wife present, and 34.1% were non-families. 30.2% of all households were made up of individuals and 9.3% had someone living alone who was 65 years of age or older. The average household size was 2.20 and the average family size was 2.64. The median age in the town was 44.3 years. 19.7% of residents were under the age of 18; 7.4% were between the ages of 18 and 24; 24.1% were from 25 to 44; 33.4% were from 45 to 64; and 15.3% were 65 years of age or older. The gender makeup of the town was 52.5% male and 47.5% female.
2000 census
As of the census of 2000, there were 390 people, 169 households, and 111 families residing in the town. The population density was 192.8 people per square mile. There were 188 housing units at an average density of 92.9 per square mile. The racial makeup of the town was 94.36% White, 3.33% Native American, and 2.31% from two or more races. Hispanic or Latino of any race were 0.26% of the population. There were 169 households out of which 33.1% had children under the age of 18 living with them, 50.9% were married couples living together, 9.5% had a female householder with no husband present, and 34.3% were non-families. 29.0% of all households were made up of individuals and 8.3% had someone living alone who was 65 years of age or older. The average household size was 2.31 and the average family size was 2.86. In the town, the population was spread out with 26.2% under the age of 18, 5.4% from 18 to 24, 30.3% from 25 to 44, 27.7% from 45 to 64, and 10.5% who were 65 years of age or older. The median age was 38 years. For every 100 females, there were 107.4 males. For every 100 females age 18 and over, there were 102.8 males. The median income for a household in the town was $36,719, and the median income for a family was $44,250. Males had a median income of $26,500 versus $18,750 for females. The per capita income for the town was $17,206. About 8.4% of families and 7.4% of the population were below the poverty line, including 8.7% of those under age 18 and 16.7% of those age 65 or over.
