This is fair. One example is the discrete math course (called Math for CS). The MIT course is widely agreed (in the Discord server anyway) to be harder to follow than even just a good discrete math textbook, but there are no high quality discrete math textbooks that are free and open source (as least that I know of).

I’m doing OSSU, and I went for “Discrete Mathematics with Applicstions” by Susanna Epps. Cost $50 for a used textbook. Technically I’m deviating from the curriculum but I’d say I’m still “doing OSSU”