There’s an original poster for each post. When you’re talking in one post about another post, there is another original poster from the other post. What you call that other original poster is up to you, but OOP is what the BORU sub settled on.
The person who posts a repost is an OP for that specific post, to differentiate them from the other commenters in the post. It’s not like Original Content, which his the root source of all reposts, it’s just a term that (automatically, by the site’s own labels) refers to the person who made that specific post, regardless of its content.