Oceanside is a beach city in the North County area of San Diego County, California. The city had a population of 174,068 at the 2020 United States census, up from 167,086 at the 2010 census, making it the most populous city in the North County region of San Diego. The city is a popular tourist destination, owing to its historic landmarks, beaches, and architecture.

Oceanside's origins date to 1798, when the Spanish founded the village of San Luis Rey with the establishment of Mission San Luis Rey de Francia by Fermín de Lasuén. During the Spanish period, Mission San Luis Rey grew to be the largest of all the Californian missions, but following the Mexican secularization act of 1833 the mission and its community declined. Following the U.S. conquest of California, the former mission lands were developed into an oceanfront resort and the community's name gradually changed to Oceanside.

Originally inhabited by Native Americans, Oceanside was first settled by the Luiseño peoples, or the Payomkawichum. The city of Oceanside sits on the locality on the San Luis Rey River that the Luiseño called Tacayme, which contained the villages of Qée'ish (Keish) and 'ikáymay (Ikamal), in the San Luis Rey Mission area, Wiyóoya (Wiawio) at the mouth of the river, and Wi'áasamay (Wiasamai) and Waxáwmay (Wahaumai) at Guajome.

The first European explorers arrived in 1769. Spanish missionaries under Father Junípero Serra founded Mission San Luis Rey de Francia on a former site of a Luiseño Indian village on the banks of the San Luis Rey River called Keish according to the Portolà expedition who visited in the 1769.

In the early 19th century, the introduction of farming and grazing changed the landscape of what would become Oceanside.

Following the Mexican secularization act of 1833, the Californian missions were nationalized, transferring their ownership from the Franciscan Order of the Catholic Church to the Mexican Government. A mission administrator was appointed to each mission to oversee the process of secularization, the removal of the priests, and the selling or granting of the mission lands to private citizens.

At Mission San Luis Rey, Padre Antonio Peyrí had presided over the mission for 34 years when soldiers forced his removal in late 1833. Captain Pablo de la Portillà was appointed as administrator of San Luis Rey. Following the departure of Padre Peyrí, relations between the Luiseños and the new civilian administration hugely deteriorated, resulting in their famed refusal to work for Captain Portillà in 1835.

In the late 1850s, Andrew Jackson Myers, an emigrant from Illinois, lived in San Joaquin County, but he returned in the late 1880s and lived in San Luis Rey. In 1882 Myers moved on the land that was the original townsite for Oceanside. A patent for the land was issued in 1883 by the federal government. It was incorporated on July 3, 1888. The city hall as of the early 21st century stands on the former Myers homestead.

The town post office contains an oil-on-canvas mural, Air Mail, painted in 1937 by Elsie Seeds. Federally commissioned murals were produced from 1934 to 1943 in the United States through the Section of Painting and Sculpture, later called the Section of Fine Arts, of the Treasury Department.

Oceanside continues to be known as a vacation home market.

In the 2010s, several mid-rise and high-rise housing and lodging projects were completed in the downtown area, with more to come. In 2021, two large Hyatt resorts were completed, bringing more tourists to the city.

According to the United States Census Bureau, the city has a total area of 42.2 square miles (109 km2), of which 41.2 square miles (107 km2) is land and 0.9 square miles (2.3 km2), comprising 2.23%, is water.

Traveling north on Interstate 5, Oceanside is the last city before Orange County. As the crow flies, it is roughly the same distance from Aliso Viejo as it is to downtown San Diego.

Together with Carlsbad and Vista, it forms a tri-city area.

Oceanside experiences a mild semi-arid climate (Köppen climate classification: BSk) that is significantly tempered by maritime winds and the cool currents off the shoreline. The average high temperatures range from 64 to 77 Â°F (18 to 25 Â°C), while the average low temperatures range from 45 to 64 Â°F (7 to 18 Â°C).

The 2010 United States census reported that Oceanside had a population of 167,086. The population density was 3,961.8 inhabitants per square mile (1,529.7/km2). The racial makeup of Oceanside was 109,020 (65.2%) White, 7,873 (4.7%) African American, 1,385 (0.8%) Native American, 11,081 (6.6%) Asian (3.4% Filipino, 0.7% Japanese, 0.7% Vietnamese, 0.6% Chinese, 0.4% Korean, 0.2% Indian), 2,144 (1.3%) Pacific Islander, 25,886 (15.5%) from other races, and 9,697 (5.8%) from two or more races. Hispanic or Latino of any race were 59,947 persons (35.9%).

The Census reported that 166,150 people (99.4% of the population) lived in households, 802 (0.5%) lived in non-institutionalized group quarters, and 134 (0.1%) were institutionalized.

There were 59,238 households, out of which 20,486 (34.6%) had children under the age of 18 living in them, 30,201 (51.0%) were opposite-sex married couples living together, 6,947 (11.7%) had a female householder with no husband present, 3,111 (5.3%) had a male householder with no wife present. There were 3,504 (5.9%) unmarried opposite-sex partnerships, and 472 (0.8%) same-sex married couples or partnerships. Of the households 14,117 (23.8%) were made up of individuals, and 6,161 (10.4%) had someone living alone who was 65 years of age or older. The average household size was 2.80. There were 40,259 families (68.0% of all households); the average family size was 3.32.

The population was spread out, with 39,817 people (23.8%) under the age of 18, 19,028 people (11.4%) aged 18 to 24, 45,797 people (27.4%) aged 25 to 44, 40,943 people (24.5%) aged 45 to 64, and 21,501 people (12.9%) who were 65 years of age or older. The median age was 35.2 years. For every 100 females, there were 97.4 males. For every 100 females age 18 and over, there were 95.0 males.

There were 64,435 housing units at an average density of 1,527.8 units per square mile (589.9 units/km2), of which 34,986 (59.1%) were owner-occupied, and 24,252 (40.9%) were occupied by renters. The homeowner vacancy rate was 2.2%; the rental vacancy rate was 6.2%. Of the population 97,645 people (58.4%) lived in owner-occupied housing units and 68,505 people (41.0%) lived in rental housing units.

As of the census of 2000, there were 161,029 people, 56,488 households, and 39,259 families residing in the city. The population density was 3,967.2 inhabitants per square mile (1,531.7/km2). There were 59,581 housing units at an average density of 1,467.9 units per square mile (566.8 units/km2). The racial makeup of the city was 66.4% White, 6.3% African American, 5.5% Asian, 1.2% Pacific Islander, 0.4% Native American or Alaskan Native, 0.1% from another race alone, and 3.2% from two or more races. Hispanic or Latino of any race was 30.2%.

In 2000, there were 56,488 households, out of which 35.0% had children under the age of 18 living with them, 54.1% were married couples living together, 11.0% had a female householder with no husband present, and 30.5% were non-families. Of all households 22.7% were made up of individuals, and 10.2% had someone living alone who was 65 years of age or older. The average household size was 2.83 and the average family size was 3.33.

The age distribution of Oceanside in 2000 was as follows: 27.6% under the age of 18, 10.2% from 18 to 24, 31.0% from 25 to 44, 17.6% from 45 to 64, and 13.6% who were 65 years of age or older. The median age was 33 years. For every 100 females, there were 98.0 males. For every 100 females age 18 and over, there were 96.1 males.

The median income for a household in the city was $46,301, and the median income for a family was $52,232. Males had a median income of $34,772 versus $27,962 for females. The per capita income for the city was $20,329. About 8.2% of families and 11.6% of the population were below the poverty line, including 16.2% of those under age 18 and 5.7% of those age 65 or over.

According to the city's 2017 Comprehensive Annual Financial Report, the top employers in the city are:

The city operates under the council-manager form of government. The city council consists of four members elected from districts and one mayor elected at large. The current council is composed of: Esther Sanchez (mayor), Ryan Keim (deputy mayor)(District 3), Peter Weiss(District 4), Eric Joyce(District 1), and Rick Robinson(District 2).

The current city treasurer is Victor Roy. The current city clerk is Zeb Navarro.

The City of Oceanside is a full-service city. It provides police and fire safety, water and sewer services. The city has a municipal airport, a small craft harbor, one of the longest wooden piers in the west, golf courses, swimming pools, numerous parks, community centers, and extensive palm-lined beaches. Oceanside Public Library is the city's public library system, with the main branch located in the civic center.

In the California State Legislature, Oceanside is in the 38th Senate District, represented by Democrat Catherine Blakespear, and in the 74th Assembly District, represented by Republican Laurie Davies.

In the United States House of Representatives, Oceanside is in California's 49th congressional district, represented by Democrat Mike Levin.

Residents of Oceanside may attend schools in the Oceanside Unified School District, Vista Unified School District, Bonsall Union School District, Carlsbad Unified School District, Fallbrook Union High School District, or Fallbrook Union Elementary School District, depending on their address. The Oceanside Unified School District provides instrumental music programs in grades 4–12.

The Oceanside Unified School District has two comprehensive high schools, El Camino High School off Rancho Del Oro and Oceanside High School off Mission Avenue. High school students are also served by Ocean Shores Continuation High School and Clair Burgener Academy. OUSD has 24 schools plus three charter schools, including the School of Business and Technology, and two brand new schools, Louise Foussat Elementary School and Cesar Chavez Middle School, that opened in the Fall of 2007. Cesar Chavez Middle School, which is on the corner of Frazee and Oleander, will be built on 14 acres (5.7 ha), house 11 buildings totaling 84,000 square feet (7,800 m2), and will serve 1,000 6th–8th grade students.

The other school, Louise Foussat Elementary School, located on Pala Road, is built on 12.6 acres (5.1 ha) of land with 35 classrooms totaling 54,490 square feet (5,062 m2) and can accommodate 800 students.

A free, independent public charter school, Scholarship Prep, is a new TK-8th grade school which was authorized to open in August 2017 and is sited at the historic San Luis Rey Mission. It offers a full educational experience for Oceanside students.

The Oceanside Transit Center is a major railway interchange for long distance and commuter rail services. Amtrak's Pacific Surfliner provides service to San Luis Obispo via Los Angeles and San Diego. Metrolink's commuter rail service connects Oceanside to many parts of Greater Los Angeles, while Coaster commuter rail serves northern and central San Diego County. The Sprinter hybrid rail service connects Oceanside to Escondido.

Oceanside Municipal Airport (OCN) is mainly used as a general aviation field. The nearest international airport is San Diego International Airport, connected to Oceanside via Amtrak and Coaster rail services.