The beauty of OER is that depending on the license permissions, you can copy open content, adapt it for your own purposes, and thereby build on the knowledge of others! Before creating an OER project from scratch, do some research about what's already out there that you can build from. Below find some great places to start your search.
Repositories enable you to search for all types of OER: textbooks, assignments, syllabi, slides, and more
OER Commons and MERLOT are helpful examples with strong search and filtering
Google Images & YouTube* (*Filter for Creative Commons)
Major publisher, peer reviewed
Includes ancillary materials
Great search portal
Includes professor reviews
Learning Matters Assignment Library: Openly licensed assignments for students made by LaGuardia colleagues
First Year Seminar Curricular Resources: Resources for FYS in the various disciplines
CUNY Academic Works OER: Openly licensed works by CUNY colleagues