Wikipedia's opening paragraph in their article on Open Educational Resources (OER) sums up the definition of OER well:
Open educational resources (OER) are freely accessible, openly licensed text, media, and other digital assets that are useful for teaching, learning, and assessing as well as for research purposes.
I collect below links to some OER materials related to math, statistics, and computer science.
Probability and Bayesian Modeling by Jim Albert and Jingchen Hu
STATS 1201 at Columbia University by Wenda Zhou in the summer of 2017
Charles Frye's blog
An interactive site from Brown University about statistics: Seeing Theory
Rob J. Hyndman's Facts and fallacies of the AIC
David Anderson and Kenneth Burnham's AIC Myths and Misunderstandings