Quality: It's a common misconception to correlate free with low quality or not to trust the quality of a free resource. Many OER resources are peer-reviewed. And there are also assessment/rating tools that you can refer to when selecting a resource.
In summary, the quality can be evaluated and assured by several means, starting by Word of Mouth to Peer Review, user comments and ratings, or institutional procedures for quality assurance. Librarians are also available for support.
Accessibility: Ensure that any resource you adopt or create remains accessible to students after the end of the course. Refer to the IT department at your institution for technical support in this matter.
Sustainability: With many OER initiatives being launched, and which has lead to competition for funding, it is essential for institutions to look into different revenue models to ensure the long term stability and viability of their initiative.
Licensing: Not all people are familiar with licensing and Creative Commons. When using an OER, always check the license. Many are licensed "CC-BY", which requires you to give attribution to the author(s) of the resource, but there are other licenses used. check the Licensing tab in this guide.
Adapting: Some might have few technical skills and find it difficult to edit or create a resource to match their requirements. In addition, specific softwares might be required for some resource. IT and Librarians provide support in this matter.
Language: Some OER might be available in the needed language, or are customized to a different culture.
Time: Creating or adapting/adopting an OER could take some time, in terms of reviewing the content and making sure it fits the learning outcomes of the course. Time is also needed to keep the resources updated and current.