Maybe my definition of Mary Sue is too broad. The part that first brought the idea to mind is where he takes over diplomacy from Hinjo and does better at it, despite Hinjo being the trained diplomat on the team. Yes, Hinjo is inexperienced, so it's not entirely without justification, but even so it's fairly classic Sue behaviour to muscle in on what is explicitly another character's turf and do it better. And then to add insult to injury, he explains to Hinjo why that technique was successful where Hinjo's wasn't, because clearly that needed doing.
Indeed it seems at times that the other charactes only exist in order to give O-Chul opportunities to demonstrate how great he is. Mrs. Kapoor does solve a couple of problems that O-Chul couldn't, but there's also that sequence where she basically opens a conversation with "So O-Chul, I'd really like to hear some exposition of your background!" to give him the cue to tell us his childhood story.
And throughout the story he comes across as pretty much perfect. He's the best officer his commander has available. His diplomacy skills are unparalleled. He wins round the Sapphire Guard, causing them to mutiny against their commander - including Miko, the commander's protégé - in a matter of minutes. He has the best moral compass of anyone in the story including the actual celestial. And at the end he's the one who has to clean up the order of paladins. I understand that he's meant to be a model paladin but I can't see anything really approximating a character flaw or vulnerability in this story. The only thing he does which is even close to an error of judgment is tying up the hobgoblins, but that was both entirely reasonable in the circumstances (so doesn't really qualify as a mistake) and ends up having no negative consequences anyway.
Maybe I missed or have forgotten something, but the only character development for him happens before this story actually starts, which for an origin story seems strange.