The name itself is a portmanteau of common internet shorthand:
In design communities (like those on DeviantArt or specialized Discord servers), creators often bundle textures, brushes, or fonts into archives. If a user compiles a "best of" pack from various artists, they might name the file credits_to_op.rar to signify they don't claim ownership of the individual assets. credits_to_op.rar
A standard "nod" to the Original Poster (OP), used to acknowledge that the content inside was created by someone else and is being reshared or archived. The name itself is a portmanteau of common