On the conference I attended both oral sessions and symposia had topics and speakers in a row so it didn't become clear to me where exactly is the difference? Is it in the duration of the lectures or the expected audience size or something else?
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.