It has been suggested that open educational resources (OER) can lower cost and lead to greater flexibility; however, while there has been significant investment in opening up content there have been few studies looking at how these resources are perceived by those who might use them. This article contributes to fill a gap in our knowledge of how K-12 educators teaching in face-to-face, online and blended contexts currently think about and use OER. It is part of the research carried out by the Hewlett-funded OER Research Hub (OERRH) Project to examine the impact of OER on teaching and learning practices. The authors report findings from an international survey of over 600 teachers who answered a set of attitudinal and behavioural questions in relation to how they use OER, what types of OER they use, and what influences their selection of content, in addition to the purpose, challenges and perceived impact of OER in the K-12 classroom. The research highlights how teachers adapt rather than simply adopt OER, suggesting a strong connection between OER use and personalized learning, and argues that mainstreaming OER in K-12 education is not only a matter of raising awareness but of changing teachers' habits.