Because they are used so interchangeably in teaching, here is a bit of an explainer.
OER is Open Educational Resources and refers to all the resources faculty uses to teach a class. This usually means the textbook but can refer to readings, videos, or anything else needed to teach that are free to the student and usually available online in some format. There can be some things that do need to be purchased, such as calculators, lab coats, and goggles.
ZTC is Zero Textbook Cost and refers to all the readings that are digital and free of cost. Students want to know if they have to buy a book or not and this makes more sense to them. ZTC was adopted by the State of California for language referring to OER and is used in the schedule
LTC is Low Textbook Cost and refers to a textbook is a course designation of class sections that use texts and/or materials that cost a total of $40 or less combined.
Other considerations:
What is OER? (Video: CC BY 4.0 Music: The Zeppelin by Blue Dot Sessions: CC BY NC 4.0)