This work proposes the Reservoir Risks Ontology (ResRiskOnto), an application ontology for risks in the oil and gas industry associated with the petroleum reservoir domain. ResRiskOnto s building blocks are terms dominated by reservoir professionals, so that it can be easily adopted in future risk documentation. ResRiskOnto is developed having at its center the concept of Risk Events. Each event has a set of possible Participants, that have its Characteristics manifested by the event. The ontology provides a total a set of 97 terms, 29 of which are derived from the Risk Event class. To develop the ResRiskOnto, we conducted a semantic analysis of documents that contain over 2500 reservoir-related risks described in natural language. This repository is the result of hundreds of risk assessment workshops in oil and gas projects, conducted in over ten years in Petrobras. This ontology is founded on the principles of the Basic Formal Ontology (BFO), a top-level ontology designed to describe scientific domains. One of BFO s most distinct characteristic is its commitment to Realism, a philosophical view of reality in which its constituents exist independently of our representations. On the domain-level, reservoir entities are described under the principles of the GeoCore Ontology, a core ontology for Geology. To validate the ResRiskOnto we annotate our risk documents repository with the ontology s entities and relations, developing a model that recognizes named entities and extracts the relations among them. Our contribution is an application ontology that allows semantic reasoning over the risk documents. We also expect to provide (i) a basis for data modelling in the case of reservoir-related risks; and (ii) a standard for future risk documentation in the reservoir domain.