OC stands for "original content" and the feature is a hold-over from how mastodon does things. the idea being that you mark it if you yourself made it and are not reposting it.
Yeah but that's just someone manually writing [OC] lol. some subreddits had link flair to mark posts as OC though. Here on kbin we have "link flair" as "badges". but also an explicit "OC" marker feature. Having that OC marker feature is something from mastodon.
This is semantics...people use a convention of [original content] on reddit, whether it's an official implementation or not. It's like before Twitter got official retweets. I didn't realize there was a flair/badge for this on kbin until just now, but it feels like an extension of that manual function. In spirit, it exists on reddit.