Abnormal situation management (ASM) involves timely identification and mitigation of any significant departure of the process from an acceptable normal range of operation. Ineffective ASM has significant safety and economic impact on the industry. In this work, an intelligent operator decision support system, called Op-Aide, has been developed to assist the operator in quantitative diagnosis and assessment of abnormal situations. Op-Aide has been developed based on an open, modular, blackboard-based architecture. In this approach, independent modules provide data acquisition, process monitoring, fault diagnosis, and situation assessment capabilities. The implementation of Op-Aide is done in G2, MATLAB and C. Its application is illustrated on the simulation of Amoco Model IV fluidized catalytic cracking unit (FCCU).