If the US were to hit Pyongyang with a nuke from an Ohio Sub, would Japan, S. Korea, or China see fallout?

China and Japan would definitely see fallout from a weapon 1 megaton in size or larger.

The exact pattern would depend on the winds that day but the jet stream would definitely get a hold of some fallout and fling it into Russia, Japan, and on into North America.

When the Fukushima reactors cracked open after the earthquake and tsunami in Japan, radioactive particles were detected all the way across the pacific ocean in North America. Of course the amount of radiation was negligible - it barely registered. That was from an incident that didn't include an explosion.

A nuclear explosion would most certainly throw tons of dust and dirt into the upper atmosphere where it would come down over several thousand square kilometers.

Nukes have been limited to 100 kt ever since ballistic missiles became very accurate, 10 - 50m CEP...

China and South Korea will definately see fallout, even parts of russia and maybe japan.