It's just as important to evaluate OER as it is to evaluate a commercial textbook. In the case of OER, we can't always take the publisher's word for it. This page is dedicated to providing resources to assist faculty and students with evaluating OER resources for quality, content, usability, sustainability, accessibility, and other issues.
"When using static, traditional textbooks, most districts rely on publishers to ensure that the materials are high quality and aligned with rigorous college and career ready standards. In contrast, when you transition to openly licensed educational resources, the onus is on you to ensure that the resources are of a high quality and provide the learning experiences necessary to exceed learning goals for all students. This is a serious, new responsibility for a district that is accustomed to buying proprietary resources from third parties. #GoOpen Districts thoughtfully and deliberately create quality assurance processes with multiple checks to ensure that materials match the needs and expectations of all stakeholders." (https://tech.ed.gov/open/districts/launch/phase4/)