Personally I wouldn’t have marked it. The idea of marking things as “[OC]” really started with art stuff and things that’d be considered “self-promotion”. Just chatting or posting text questions/comments/etc. aren’t really expected to get marked. I don’t think there’s anything wrong with it per se, but from the internet circles I’m from it’s not the norm; only to explicitly mark art/videos/software you made. It’s typically used in art communities to distinguish making something vs sharing it.

So yes, you’re getting the right vibe. Imagine you have a magazine where people post art. The norm is to share art they found elsewhere. while some people make their own art and wish to share it. so it’s marked as OC.

On reddit this became the norm because reddit’s focus was content aggregation, not self promotion, so self promotional stuff (OC) was discouraged unless specifically marked. Here on kbin I think it’s probably less important, though some magazines might want to make that distinction.

It’s also useful for twitter/mastodon/microblog stuff since there’s a culture of just reuploading/resharing pics, and some might want to specifically specify art created by the account.