Open educational resources (OER) are increasingly attractive options for reducing educational costs, yet controlled studies of their efficacy are lacking. The current study addressed many criticisms of past research by accounting for course and instructor characteristics in comparing objective student learning outcomes across multiple sections of General Psychology taught by trained graduate student instructors at a large research-intensive university. We found no evidence that use of the OER text impeded students' critical thinking compared to use of a traditional textbook, even after accounting for instructor characteristics. To the contrary, we found evidence of a slight increase in content knowledge when using an OER text. Importantly, this effect was driven by improvements from both our lowest-performing students and our highest-performing students. Moreover, student learning outcomes were not influenced by instructor experience, suggesting even novice instructors fared well with OER materials. Finally, students from traditionally underserved populations reported the lower cost of the book had a significantly higher impact on their decision to enroll in and remain enrolled in the course.