When making my deities, I've always given them just 3 or 4 domains: Alignement + 2. Any more than that, they have to take the Extra Domain salient divine ability, and Extra Domain has divine rank 5 as a prerequisite.
Chaos and Evil are of course mandatory; reflecting his alignment. Going strictly by the PHB, I'd also give him Trickery and Death.
If you're using BoVD, the Demon domain is an obvious choice. He is a demon, after all. Loose trickery.
FRCS had the Undeath domain, which -for Orcus- is a suitable replacement for Death, IMHO.
To sum up; if Orcus was a deity in MY campaign, he'd have Chaos, Demon, Evil & Undeath. If he had more than five divine ranks, he might also have Trickery.
Just my 2 cents, of course.