"There comes a point when all good people have to walk away from garbage like that," he said.
Casserly said O'Reilly's accusation that he was "emotionally disturbed" was "probably the only accurate thing he's ever said".
"He's obviously been speaking to some of the medical professionals that I work with, so I'd like to know how he knows."
The preview of Killing Kennedy was meant to highlight O'Reilly's penchant for writing about dead presidents, rather than imply the pundit genuinely wanted Obama dead, he said.
"I was just saying if there was a book, he would be the man for the job. He'd be the obvious candidate, if he was up to the task."
He believed O'Reilly was exaggerating for dramatic effect in the segment criticising him.
"He's theatrical and that theatrical approach is to exaggerate things. He probably thought about it for all of 10 seconds."
- nzherald.co.nz