OTC or Rx Drugs?

What determines if a new drug will be over the counter (OTC) or require a prescription (Rx)? Other than something obvious like a narcotic, how does the FDA make this call?

I suppose one could start by checking the FDA’s own site for How Drugs are Developed and Approved. Weird notion, I know.

I do know of at least one drug that is prescription in Canada and OTC in the US and at least for which the reverse is true. In a lot of cases, it makes little or no sense. I once read that if aspirin had been discovered in the 20th century, it would likely be prescription only.