If you're into that niche, Archive of Our Own is basically the central hub these days. The tagging system there is incredibly detailed, so you can navigate directly to 'One Piece' and then filter for the 'Futanari' tag, often paired with character or relationship tags. The sheer volume of work means there's a wide range, from quick, purely smutty pieces to longer, plot-heavy stories that weave the fantasy elements into the existing world-building. It's a community-driven space, so the quality varies, but the kudos and comment counts are usually a reliable guide to what's currently trending within that specific subset of the fandom.
Tumblr still hosts a surprising amount of this content, though it's more of a scattered network of individual blogs. Finding it requires a bit more digging—searching through specific tags like '#opfic' or '#onepiecefanfiction' combined with more explicit indicators. Writers often use it to post snippets or link out to their full stories on Ao3 or other sites. It feels more personal and direct, sometimes offering a rawer, less polished style that has its own appeal compared to the more formal archive structure.
For dedicated fanfiction apps, platforms like Wattpad can have some finds, but the tagging is less precise, so you'll need patience. The search algorithms there often prioritize overall popularity over niche specificity. I've found that the stories leaning into 'spicy' content on those wider platforms sometimes soften the more extreme edges of the fetish, blending it more with conventional romance or adventure plots, which can be interesting in its own right. Ultimately, your preference for a vast, searchable library versus a more intimate, blog-style feed will point you to the right spot.