The California State Water Resources Control Board (State Water Board) initiated the Oil and Gas Regional Monitoring Program (RMP) to assess effects of oil and gas development on groundwater designated for any beneficial use. The U.S. Geological Survey (USGS) is the technical lead in conducting the RMP through the California Oil, Gas, and Groundwater (COGG) Program, working in cooperation with the State Water Board, and in partnership with other State and local agencies. The USGS collected and analyzed groundwater for the Placerita Oil Field study area, the area within the Placerita Oil Field administrative boundary and the surrounding three-mile buffer zone in Los Angeles County, California. Sixteen groundwater samples were collected from June 2018 through November 2018 from three public-supply wells, two monitoring wells, three irrigation wells, two oil-field water supply wells, and one domestic well. Samples were collected using established groundwater data-collection protocols and procedures. Samples were analyzed for (1) water-quality indicators, (2) major and minor ions, (3) nutrients, (4) trace elements, (5) volatile organic compounds (VOCs), (6) naturally-occurring radioactive material (radium isotopes), (7) geochemical and age-dating tracers, (8) dissolved organic carbon (DOC) characteristics, (9) dissolved standard and hydrocarbon gases (methane through hexane), (10) dissolved noble gases and atmospheric gases, and (11) low molecular weight organic acids. In total, 229 constituents, water-quality indicators, or properties were measured. Quality-control (QC) samples were used to assess the quality of laboratory results. QC samples were collected for all oil fields assessed by the Regional Monitoring Program during the general time period (within three months) of groundwater samples that were collected for the Placerita Oil Field study area and include four replicates, three field blanks, one equipment blank, four source-solution blanks, three laboratory-matrix spikes, and two samples collected to verify results of environmental samples that arrived at the laboratory above the recommended temperature for preventing degradation of VOCs and nutrients.