The study is grounded in the fact that in Southeast Asia, there are few studies on the effects of authentic, inquiry-based learning, or instruction in the field of Science education. This study investigated the effects of the open inquiry learning model in Physics on the concept and 21st century skill attainment, and learning attitudes of grade 12 students of a state university in the Philippines. The study involved a pre-test-posttest experimental design using quantitative approaches. Normalized Hake gain was used to determine the effectiveness of the open inquiry learning model in enhancing the concept attainment of students. Non-parametric test, particularly, Wilcoxon signed ranks test, determined the significant difference between the pre and post-test scores. Net shift in pre-test and post-test scores in Colorado Learning Attitude about Science Survey (CLASS) identified a shift in the learning attitudes of students. The difference between the pre-test and post-test scores of students was found to be significant (Z=-3.927, p=0.000<0.05; Z=-3.387, p=0.001<0.05). Students achieved high Hake gain (0.82). There was also a positive shift in the learning attitudes of students. Thus, the open inquiry learning model is effective in improving the conceptual understanding, 21st century skills, and learning attitudes of students. Because of its positive effects on students’ holistic learning, further promotion of this learning pedagogy is needed, especially in the Philippines setting. A series of professional development programs anchored on this learning pedagogy may be launched to train teachers and pre-service teachers.