Domain Ontology Engineering develops, validates, and maintains the formal models that provide OntoKG with a shared understanding of the Oil and Gas domain. This work defines industrial entities, their properties, and their relationships, covering equipment, processes, physical and functional connections, and operational events. Guided by use cases, competency questions, and domain expertise, the area combines conceptual modeling, reuse and integration of existing ontologies, consistency checking, documentation, and version management. Its central role is to establish a common semantic foundation for interpreting information from operational systems, corporate databases, and technical documents, ensuring that data integration reflects domain meaning rather than simply matching names or identifiers. This foundation guides knowledge graph construction and enrichment and supports more precise retrieval and contextualized answers from LLM applications, contributing to the identification and analysis of incidents that may affect production capacity.