Can Open Campus classes give a realistic idea of normal university teaching?

They can provide some insight, but a mock class alone cannot show exactly what ordinary university classes are like. It can help you observe the instructor’s explanation style, the pace of the class, and the atmosphere among participants. However, the time and content may be shortened or specially adjusted, so the actual difficulty, amount of coursework, and interaction among students may be different. It is also useful to check publicly available syllabi and course information.