The consideration of the Design Extension conditions (DEC) has been introduced with great importance to enhance the plant safety capabilities. DEC have low occurrence probabilities but high consequences, and relatively complex scenario progression compared to Design Basis Accidents (DBA). This study suggests the estimation methodology of the DEC safety margin and the conditional core damage frequency in accordance with a DEC initiating event by combination of Deterministic and Probabilistic Safety Assessment (DSA and PSA). First, the safety limits were revisited and modified to be less conservative for DEC. Second, the safety analysis of the possible scenarios from one initiating event was performed by the best-estimate DSA approaches to obtain the actual values under accident conditions. In this process, the aleatory uncertainties were treated to generate various scenarios from one initiating event based on PSA. We introduced State Change Time (SCT) concept to quantify the aleatory uncertainties. SCT was defined as the time when the status of a function changes success to failure or failure to success. SCT of each safety system was obtained through the fault trees and the reliability data of PSA approaches, and it can describe the time-related transient of the random status. Moreover, to overcome the limitation of PSA such as high dichotomy and user-dependency, a random sampling method was introduced to automatically generate various possible sequences based on the probabilistic distributions of SCT. Latin Hypercube Sampling method and Wilks' formula are adopted to decrease the computational effort: 124 runs are required to get the output and estimate the representative values. With this method, the safety margin and conditional core damage frequency of SBO in OPR-1000 is evaluated. The best-estimate safety limit related to cladding integrity is selected and thermal-hydraulic calculation is performed MARS-KS code. In case of SBO without AAC operation, it is found out that the probability exceeding the safety limit of peak cladding temperature reaches 17% which is higher than 5% required in the DBA analysis. In case with AAC operation, the safety margin and the conditional core damage frequency are 1007.38 degrees C and 0.0017, respectively. The sensitivity studies of the sample number were performed with 3 random seeds. The results showed that 124 cases were enough to evaluate the safety margin, but the further study with higher number than 124 cases may be needed to evaluate the conditional limit exceedance frequency in the case of SBO.