In Japan, the fandom communities generally hold a lot of respect for their favorite creators. When these creators release original characters and do not want fan art made of them.
But sometimes the author is totally fine with fan works. In this case, they’ll tag their OCs with 描いてもいいのよ (kaite mo ii no yo, lit. “it’s okay if you draw this”).
Furthermore, fans of the originals can tag their work with 描いてみた (kaite mita, lit. “I tried drawing it”) and add it as an Image Response so that all fan efforts are in one place. In this way, pixiv helps foster positive creative communication between artists of all kinds!