There's actually a lot of ways people tag cosplay, and I don't think there's a 'standard this is right' way on this site. Some tag the character in the cosplay, and the target of the cosplay. Sometimes the target of the cosplay is "implied". For now, tagging both the cosplayer and the cosplayed character doesn't seem like any kind of harm - if you want just a particular character without cosplay you can `-cosplay`.
If you'd like to formalize this, then a dedicated thread in the [Tagging Discussion forum](https://furbooru.org/forums/tagging) would be a much better place to discuss it than on fanart.
[@Anonymous \#7F5E](/images/302680#comment_35944)
Please tag the character in the cosplay - for now I've added oc tags plus `oc needed`.