The rising prevalence and complexity of online harms, particularly those disproportionately affecting women, demand urgent, interdisciplinary, and socio-technical responses. Despite increasing awareness and policy action, current responses remain fragmented across disciplinary silos, limiting the development of cohesive and effective interventions. This paper presents our efforts to co-create a comprehensive Gender-aware Ontology of Online Harms as a shared knowledge structure to bridge disciplinary perspectives and inform practice across sectors, including policing, law, behavioural science, and technology. Our ontology aims to capture the full spectrum of gender-based online harms, their sociotechnical enablers and inhibitors, and their manifestations in online contexts. The development process employs a co-creation approach grounded in collaborative ontology engineering and iterative stakeholder engagement. It has been driven by discipline-specific Personas and Competency Questions to ensure relevance, usability, and impact across diverse domains. We argue that this work represents a crucial step toward formalising a shared understanding of online harms to support policy reform, technological innovation, and survivor support.