OC stands for "original content". the idea is that if it's your own created content you check the box. if you're reposting or sharing something you didn't make, then you don't check it.
iirc it's included because of mastodon and other early activitypub groups having that sort of culture. so yes, in theory it should sync I think? similar to the nsfw/18+ flag.
If you're familiar with how they do things on reddit, it's a similar ideas as marking your post with [OC] and other such. basically if you draw something or make a video and share it, then mark the box. otherwise no.