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Emergency Spillway during just two days of limited overflow release resulted in concerns for the Emergency Spillway structures and the evacuation of communities downstream of the dam. In early March 2017, the Oroville Emergency Recovery (OER) design team was formed to design and implement repairs in order for the Gated Spillway chute and the Emergency Spillway to be able to safely pass reservoir inflows during the following 2017-2018 Winter Flood Season. Within a week, the OER design team had identified a number of potential repair alternatives and the associated geologic and geotechnical data required to develop the designs. This paper provides a background of the geologic conditions known prior to the incident and the subsequent development and execution of the geologic and geotechnical investigation program to support the OER. The geologic materials at the Oroville Dam site are structurally complex, with numerous cross-cutting shear zones that make predicting ground conditions difficult. The complex geology coupled with the urgent time-frame in which to develop and implement a field investigation and laboratory testing program and then provide useful high-quality subsurface geologic and geotechnical information to the OER design team presented many challenges. These challenges ranged from resource management, to assessing a wide range of conceptual and then preferred design components, to field investigation QA/QC, to expedited data analysis and interpretations needed to properly support development of the time- critical project design. By the end of the design phase of geologic exploration in July 2017, the exploration program had grown to over 100 rock-core borings, 22 seismic lines, 26 piezometers, 15 inclinometers, 68 concrete cores, trench explorations, thin section petrographic analysis of over 30 rock samples, x-ray diffraction of 6 clayey samples, and geotechnical testing of many more rock and soil samples. In order to accomplish the exploration program, consistent documentation was achieved by preparing a workplan and having an experienced full-time engineering geologist serve in a QA/QC role to ensure that the workplan was being followed. Results of the explorations were relayed to the OER design team in real time to be incorporated into the engineering design package. The geological and geotechnical exploration program was successful because of dedicated, coherent design and field teams. Keywords Oroville Dam, Boring, Emergency Spillway, Chute, Flood Control Outlet, Foundation, Gated Spillway, Geological Investigation, Main Spillway, Monolith, Emergency Spillway. 121