Geographic information systems are useful tools for analyzing and explaining the changing spatial structures of medical services. This study identified underserved areas for out-ofhospital cardiac arrest (OHCA) using a geospatial methodology and provided a visual map of the spatial imbalances of OHCA. This study used geocoding of OHCA locations, emergency medical service (EMS) incidents, and administrative districts to demonstrate a technique for identifying underserved areas for EMS-assessed OHCA. We created a 1 km hexa-grid across Jeju Island and calculated the OHCA ratio (OHCA cases/total EMS incidents) for each grid. Underserved areas were detected through a hexagrid-based geospatial analysis (Getis-Ord Gi* statistics). A total of 366,155 EMS incidents occurred during the study period. Among these, 6,806 were EMS-assessed OHCA cases. An analysis of 1,903 overlapping grids revealed that the mean distance between grids with OHCA was 28.0±15.8 km. The mean EMS-assessed OHCA ratio (EMS-assessed OHCA cases/total EMS incidents) for each grid was 0.021±0.059. Based on the Getis-Ord Gi* OHCA ratio, 118 hot spot grids (z-score≥1.96) and 19 cold spot grids (z-score≤-1.96) were identified. This study proposed a geospatial methodology to define and identify underserved areas of EMS-assessed OHCA cases using hexa-grid-based hot- and cold-spot analysis.