Orga is an artist tag associated with mostly Ghibli uploads to Danbooru from April to November of 2007 by ShinjiDude

It's unclear which account (Pixiv or otherwise) this artist alias may have been associated with.

Later artists would also come to use the name, generating subsequent confusion:

organ derwald
orga_(orgacl)
yomoyama874

Eventually a longer name (perhaps a parenthesis for emphasis) could be useful here, as well as any external sites the artist may have used in the early 2000s.