OP can refer to the original commenter in a thread, in context of a post with a lot of conversation. So OOP in the same comment refers to the original OP, the person that posted the... Post.
Usually you see that in repost subs like best of Redditor updates. There's the OP who finds the post and puts it on BORU. When people comment on that post, they are replying to OP, not the person who it is about. So if they have a comment on that post, they would refer to the author of the original post, the original original poster.
Typically subreddits that repost other posts from other subreddits ( r/BestofRedditorUpdates for example ) refer to the original poster of that post as OOP (original "original poster"), to show people that the person posting this reposted story isn't the original poster.
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chamberofcoal t1_ixoyhvm wrote
OP can refer to the original commenter in a thread, in context of a post with a lot of conversation. So OOP in the same comment refers to the original OP, the person that posted the... Post.