Critical thinking skills are one of the 21st century skills that are effectively trained by using the OR-IPA and Problem Based Learning (PBL) Model, therefore this research aims to compare the effectiveness of both. Research design used True Experiment with Randomized Subject Control-group Pre-test and Post-test with 94 pre-service physics teachers. Data collected using the critical thinking skills test and the student response sheet, and then analyzed using t-test and N-gain. The results showed: (1) the teaching instruments of OR-IPA and PBL Model have fulfilled the validity requirements (r[alpha] ~ 0.26) and reliability ([alpha] = 0.96 - 0.99). (2) Each of OR-IPA, PBL, and Conventional Model can significantly increase critical thinking skills at [alpha] = 5%, respectively with average N-gain: medium (0.60), medium (0.48), and low (0.14); with the student response of: very positive, very positive, and less positive. (3) The OR-IPA and PBL Model are effective to improve critical thinking skills, while the Conventional Model is ineffective, and the OR-IPA Model is more effective compared to the PBL Model. Implication of this research is that the OR-IPA Model can be an innovative solution to improve critical thinking skills, but there is still a need for repetitive research like this.