What is the difference between symposia and oral sessions?
This may very well depend on the conference in question, the field, etc.

In the conferences I go to, which are in the biomedical/public health realm, broadly:


Symposia are all talks addressing a unified subjection/question/theme. "Should we be using universal contact precautions to address MRSA?", "Are causal inference methods and agent-based modeling methods compatible with each other?", etc. These talks are often deliberately solicited by the organizer.
Oral Sessions are often just collections of interesting talks on a much broader subject - "Infectious Diseases", "Bayesian Methods", etc. The talks themselves are often unsolicited, and just happen to be the best/most interesting/best fitting talks selected by the organizer.