Crowd evacuation management at large venues such as airports, stadiums, cruise ships or metro stations requires the deployment and access to a Common Operational Picture (COP) of the venue, with real-time intelligent contextual interpretation of crowd behaviour. Large CCTV and sensor network feeds all provide important but heterogeneous observations about crowd safety at the venue of interest. Hence, these observations must be critically analyzed and interpreted for supporting security managers of crowd safety at venues. Specifically, the large volume of the generated observations needs to be interpreted in context of the venue operational grounds, crowd-gathering event times and the knowledge on crowd expected behaviour. In this paper, a new context ontology modelling approach is introduced. It is based on knowledge about venue background information, expected crowd behaviours and their manifested features of observations. The aim is to improve situation awareness about crowd safety in crisis management and decision-support.