Best: Yesterday, the discussion in Beng Kok was about whether or not inpatients can vote. More informal but okay: Yesterday, the discussion in Beng Kok was on whether or not inpatients can vote.
Not correct: Yesterday, the discussion in Beng Kok was over whether or not inpatients can vote.
It would be correct to say, "The argument was over whether or not inpatients acan vote."
When you use 'about', you are indicating to the listening:
'talk about the topic of; concerning this topic' 'to express location in a particular place (place =topic) BUT also 'to indicate movement within a particular area (the topic area)'.
When you use 'on', you take out the sense of 'freely moving within a particular topic area'.
'on' is used to indicate that the discussion will focus on the specific topic under discussion.
As far as people talking in a group goes, whether you say 'about' or 'on' won't stop people from dragging in extraneous information about the topic (that they think is relevant), rather than focussing on the specific topic.
I'm exaggerating, but trying to show the difference:
Topic: Treatment of Anxiety Disorders 'on topic' : Drug X has been shown to be more effective than Y.......Cognitive Behaviour Therapy is more effective than Z.
'about' : "I remember when I had my first nervous breakdown. Xanax didn't work for me, so my doctor changed me to Prozac. I think we need to give a lot more consideration to what the patient has to suffer while the doctor is finding the right medication to treat someone's anxiety."