One of the biggest concerns faculty have about OER is whether the materials are as high quality as publisher-vetted textbooks. As the open education movement grows and more faculty contribute to OER creation, quality will continue to improve.
The Academic Senate for California Community Colleges advises,
"As with the selection of textbooks, the responsibility for determining rigor and quality of OER materials rests with the faculty member. It is true that there are materials touted as OER that lack sufficient rigor, including vanity pieces, self-published materials that have never been peer reviewed, or agenda-driven corporate materials. However, there are many more available materials of high quality worth the consideration of discipline faculty. One such resource is the COOL4Ed page http://cool4ed.org, the result of the work of the California Open Educational Resources Council (COERC) which includes OER resources which have been peer reviewed by faculty in at least two of the public systems of higher education in California. The COOL4Ed page provides not only information about the texts, but reviews and peer evaluations of each of the materials evaluated. The website is a good starting point for faculty interested in integrating OER materials into their courses. In addition, if an OER resource is lacking in some way, faculty can typically modify or supplement the materials to address any deficiencies."
See ASCCC's "mythbusting" article here.
An important aspect to quality is the accessibility of materials. Since one purpose of using OER is to improve student success, it's crucial that the OER don't hinder the success of students with varying abilities. Fortunately, there are numerous groups in the open education space trying to improve the accessibility of OER.