While it will be very easy for a few faculty members teaching introductory courses to replace a commercial textbook with an OER that has been specifically designed to take its place (e.g. adopting an OpenStax textbook in its entirety), the attraction of OER is the ability to customize and expand them in response to the needs of a particular course. Therefore, those who assign OER often choose to do much more work than their peers who simply switch from one commercial text to another.
What kind of work? It depends on the particular situation, but beyond the basics of open licensing, there's researching available OER, learning how to edit and add content to them, connecting them to the LMS (Moodle), assessing their pedagogical effectiveness in the classroom, and improving them for the next time. Furthermore, the fact that assigning openly licensed resources enables new kinds of assignments to be created usually means that some course redesign takes place whenever faculty members adopt OER.
How should this work be described by faculty members and evaluated by tenure and promotion committees? This page contains resources on this important question and increasingly pressing question.