Teachers in K-12 settings often teach out of textbooks in order to provide differentiated instruction. Open educational resources (OER) might be an effective alternative to traditional textbooks for K-12 teachers because of a free access to a rich collection of open-licensed educational materials. Understanding how to encourage teachers to accept OER is thus critical. However, existing evidence mainly concerns the advantage of OER in cost reduction but overlooks the merit of openness. To fill this gap, this mixed methods inquiry, implementing the technology acceptance model, integrated qualitative and quantitative findings to obtain a comprehensive understanding of teachers' intentions of adopting OER in K-12 settings. This study found that perceived ease of use and perceived usefulness predicted K-12 teachers' intention to adopt OER with attitudes toward OER as a mediating variable. Perceived ease of use also determined perceived usefulness. Teachers' perceptions of OER adoption were also examined for additional insight into the quantitative findings. Practical implications of adopting OER in K-12 curriculum were discussed.