The purpose of this study was to assess the effect of a simulation nursing education program in terms of clinical reasoning, problem-solving process, self-efficacy, and clinical competency using the Outcome-Present State-Test (OPT) model in nursing students . The participants comprised 45 undergraduate nursing students recruited from two universities in Korea . The number of nursing students assigned to the experimental group and control group were 25 and 20, respectively . For a period of two weeks, the experimental group received a simulation nursing education program using the OPT model, while the control group received a traditional clinical practicum . The data were analyzed using prior homogeneity tests (Fisher's exact test and paired t -test); ANCOVA was performed to investigate the differences in dependent variables between the two groups . There was a significant improvement in clinical reasoning (F = 10.59, p = 0.002), problem-solving process (F = 30.92, p <0.001), and self-efficacy (F = 36.03, p <0.001) in the experimental group as compared to the control group (F = 10.59, p = 0.002). Moreover, the experimental group showed significantly higher scores in clinical competency than the control group (F = 11.07, p = 0.002). This study demonstrates that the simulation nursing education program using the OPT model for undergraduate students is very effective in promoting clinical reasoning, problem-solving processes, self-efficacy, and clinical competency.