The adoption of open educational resources (OER) in an institutional setting has been shown to reduce the cost of course materials for students, minimize inequities, and allow instructors to customize course materials. This study tapped into instructors’ perspective of limited adoption of OER in a Mathematics department at a land-grant University in the Southeastern United States. A convergent mixed-methods study was conducted to determine how Mathematics instructors used both commercial and open-license course materials, and how OER were perceived within the Mathematics department, as well as to gauge barriers to the adoption of OER. Quantitative and qualitative data were collected by a preliminary survey administered to the Mathematics department and follow-up semi-structured interviews with voluntary participants. The results show that there are numerous barriers that deter Mathematics faculty from the use of OER, including content quality, time concerns, and difficulties customizing content. Additional themes derived from the interviews were that textbook adoption for the lower-level mathematics courses is often determined by a department-level committee, but mathematics instructors have attempted to integrate OER in their teaching. The findings of this study provided practical implications for raising mathematics instructors’ awareness of OER and identifying the action plan for implementing OER in mathematics classrooms.