Recommendations for online differential geometry classes include MIT Open Courseware, a course by Prof Arezzo, and various YouTube videos for their concise format and interactive graphics. The discussion highlights the challenge of finding quality courses in this subject area. A user suggests starting with shorter, visual content before progressing to more comprehensive lectures for deeper understanding. They also mention previous study materials, including Wheeler's Gravitation and Flanders' Differential Forms, as useful resources. Overall, the conversation emphasizes the importance of a solid foundation in differential geometry for studying general relativity.