As a native English speaker, I had no idea what OH was either. I’ve seen SO for significant other and not stack overflow, and I’ve seen reference to better half not just other half. By that choice, I am left to assume this person feels they are the better half which says a lot about them.
As a native speaker, you probably scratched your head and worked out what could fit in that gap and eventually worked it out. Then you'll grumble because the other speaker didn't choose your preferred diction.
As a non native speaker you'll probably just feel upset/hopeless/angry.
From my experience, "non-native" here includes people who are "fluent".
So we arrive at the situation where my OH-SO beloved wife is fluent in English and is definitely better than me at writing clearly constructed English essays but when it comes to usage of random idioms/slang or understanding local (and foreign!) English accents I have a very clear advantage.
> By that choice, I am left to assume this person feels they are the better half which says a lot about them.
What a ridiculous assumption.
Maybe they consider themselves and their partner to be equal halves of a whole. You know, the definition of half.