We investigate Open Educational Resources (OER) in post-secondary Calculus with face-to-face instruction using web-based homework in a side-by-side comparison with Closed (Proprietary) Educational Resources (CER). Statistical analyses using multilinear regression models are developed to demonstrate several significant effects, to within a probability of 5%. Our first finding is that students' pretest scores and access dates to online homework were both significant factors in predicting first exam scores. While pretest scores were similar between the groups, students in the OER group accessed the online homework earlier in the semester, which contributed to higher first exam scores. Second, homework scores were significantly higher in the CER group, which was a significant measure of final exam scores. In understanding this result, we cite student comments suggesting the proprietary CER homework system had more resources providing help on problems. However, the differences in final course grades were not significant. We conclude from our study that the OER materials are effective, but recommend that care is taken to ensure the free materials provide a quality experience.