A very wealthy Core world. Simon and River grew up here and Simon studied and practiced medicine here as well. The planet is 'advanced' enough to have developed truly disreputable areas in the urban sprawl -- places known as Blackout Zones where loyal Alliance citizens simply do not go. The main city on Osiris is called Capital City, which is where Simon worked as a surgeon.
Osiris is the capital of the Alliance. In River's classroom scene in the movie "Serenity", Osiris is the third planet listed on the screen to the left of the teacher. Ariel may be a moon of Osiris.