In cooperation with the California State Water Resources Control Board’s oil and gas Regional Monitoring Program, the U.S. Geological Survey collected and analyzed groundwater and associated quality control (QC) samples during June 2017 – August 2017. Groundwater samples were collected from one public supply well, four monitoring wells, and nine irrigation wells located within a three- mile buffer zone overlying and surrounding the Oxnard oil field administrative boundary in Ventura County, California. Samples were collected using established groundwater data-collection protocols and procedures. Samples were analyzed for water-quality indicators, major and minor ions, nutrients, trace elements, volatile organic compounds (VOCs), polycyclic aromatic hydrocarbons (PAHs), naturally occurring radioactive material, geochemical and age-dating tracers, dissolved organic carbon (DOC), low molecular weight organic acids, dissolved standard and hydrocarbon gases, and dissolved noble and atmospheric gases. Approximately 280 constituents and water-quality indicators were measured. Quality-control samples including replicates; source-solution, equipment, and field blanks; laboratory spikes; and split samples for inter-laboratory comparisons were collected and summarized in this data release. A data dictionary was created to describe the data tables and is provided with this data release. Please refer to this data dictionary file called COGG_Data_Dictionary_OxnardGW for definitions of fields within these data files.