I'll probably regret putting my 2 cp into this, but I always understood that the thing that distinguishes a Mary Sue is that their skills (either in quantity, degree, or both) far exceed what the setting and narrative warrant. I don't think O-Chul meets that standard: his insights into the way the Hobgoblin Chief or the Paladins serving under Gin-Jun are born of what he's seen in his dozen years as a soldier.