Patient Safety (PS) standardization is the key to improve interoperability and expand international share of incident reporting system knowledge. By aligning the Patient Safety Categorial Structure (PS-CAST) to the Basic Formal Ontology version 2 (BFO2) upper level ontology, we aim to provide more rigor on the underlying organization on the one hand, and to share instances of concepts of categorial structure on the other hand. This alignment is a big step in the top-down approach, to build a complete and standardized domain ontology in order to facilitate the basis to a WHO accepted new information model for Patient Safety.