OpenEM is a large-scale, multi-structural three-dimensional geoelectrical model dataset designed to support research in geophysics, particularly in the areas of electromagnetic (EM) modeling, inversion, and machine learning applications. The dataset includes nine types of geological models that cover a wide range of resistivity distributions and structural complexities. Due to file size limitations and to facilitate convenient use in deep learning tasks, the full dataset is hosted on https://huggingface.co/datasets/ws11/OpenEM. The forward modeling code developed for OpenEM is openly available at https://github.com/WAL-l/OpenEM.