|• Total||1.3 sq mi (3.4 km2)|
|• Land||1.1 sq mi (2.8 km2)|
|• Water||0.2 sq mi (0.6 km2)|
|Elevation||10 ft (3 m)|
|• Density||1,200/sq mi (460/km2)|
|Time zone||Eastern (EST) (UTC-5)|
|• Summer (DST)||EDT (UTC-4)|
|GNIS feature ID||0615548|
Onset is located at (41.746424, -70.663251).
According to the United States Census Bureau, the CDP has a total area of 3.4 km² (1.3 mi²), of which 2.8 km² (1.1 mi²) is land and 0.6 km² (0.2 mi²) (16.92%) is water.
As of the census of 2000, there were 1,292 people, 568 households, and 323 families residing in the CDP. The population density was 461.9/km² (1,192.7/mi²). There were 910 housing units at an average density of 325.3/km² (840.1/mi²). The racial makeup of the CDP was 74.46% White, 7.74% African American, 0.23% Native American, 1.24% Asian, 13.08% from other races, and 3.25% from two or more races. Hispanic or Latino of any race were 1.78% of the population.
There were 568 households out of which 22.9% had children under the age of 18 living with them, 34.2% were married couples living together, 17.3% had a female householder with no husband present, and 43.0% were non-families. 35.0% of all households were made up of individuals and 13.9% had someone living alone who was 65 years of age or older. The average household size was 2.27 and the average family size was 2.93.
In the CDP the population was spread out with 22.4% under the age of 18, 5.6% from 18 to 24, 28.6% from 25 to 44, 24.8% from 45 to 64, and 18.6% who were 65 years of age or older. The median age was 41 years. For every 100 females there were 93.4 males. For every 100 females age 18 and over, there were 88.3 males.
The median income for a household in the CDP was $31,250, and the median income for a family was $36,739. Males had a median income of $37,629 versus $26,903 for females. The per capita income for the CDP was $18,088. About 13.0% of families and 23.6% of the population were below the poverty line, including 43.8% of those under age 18 and 14.0% of those age 65 or over.
Onset was developed in the 1880s as a summer camp meeting for Spiritualists. Many of the existing cottages in Onset were built as second homes for individuals from Boston, Taunton, Brockton and other northeastern cities who gathered to hear mediums communicate with the dead. While it was run by the Spiritualists, the village was known as Onset Bay Grove.
- "Profile of General Population and Housing Characteristics: 2010 Demographic Profile Data (DP-1): Onset CDP, Massachusetts". U.S. Census Bureau, American Factfinder. Retrieved March 27, 2012.
- "US Gazetteer files: 2010, 2000, and 1990". United States Census Bureau. 2011-02-12. Retrieved 2011-04-23.
- "American FactFinder". United States Census Bureau. Retrieved 2008-01-31.
- Moore, William D. (1997). "'To Hold Communion with Nature and the Spirit-World:' New England's Spiritualist Camp Meetings, 1865–1910". In Annmarie Adams and Sally MacMurray. Exploring Everyday Landscapes: Perspectives in Vernacular Architecture, VII. Knoxville: University of Tennessee Press. ISBN 0-87049-983-1.