Adding an OER to Canvas, or distributing it to your class in some other way, is very simple.
The most straightforward way is to simply provide a link to the textbook's page, like this:
In this class, we will be using Anatomy and Physiology by Betts, et al.. It is a free textbook available from OpenStax here.
Many OER providers also have Canvas integration for their materials. OpenStax, for instance, invites faculty to register when they adopt a textbook, which unlocks many instructor features, including Canvas "cartridges" which can be loaded into your course (see the Digital Instruction designers for details).
Many OER providers have content available in Canvas Commons. Chris Long shows how to create a course using OpenStax content from the Commons.