"OER" and "ZTC" are often used interchangeably but they are not necessarily the same thing. Here's how they're different.
Open Educational Resources (OER) carry an open license. -- Creative Commons - CC which grants permission to access OER materials for free along with the permission to modify the work in the following ways:
ZTC stands for Zero Textbook Cost and is a broad umbrella term for courses where there are no required textbook related costs. These courses may use a combination of openly licensed content, links to scholarly or professional websites, library resources, original content, or other free resources for students. In many cases, ZTC courses utilize a curated collection of many resources rather than a single text.
Some of the learning materials may cost the students zero dollars but a cost is incurred somewhere. For example, materials from Western Library's resources are considered ZTC but not OER. Why?