This may not help a lot, but I found somewhere that listed the notes in Opium as follows:
Cinnamon, Pepper, Orange, Pimento, Carnation, Jasmine, Rose, Ylang-ylang, Vanilla, Benzoin, Patchouli, Opoponax.
Horreur Sympathique (Diabolus) has "blood musk, golden honey, thick black wine, champagne grapes, tobacco flower, plum blossom, tonka bean, oakmoss, carnation, benzoin, opoponax, and sugar cane" and so has several notes in common.
Satan and Death with Sin Intervening (Salon Exhibit I) has "Opoponax, benzoin, orange blossom, mahogany, karakarounde, white tea and vetiver."
I haven't actually smelled either of these, so I don't know if they'd be close or not. From the notes alone, I'd be more optimistic about Horreur Sympathique, but there's no way to know how those notes are mixed until you actually smell them. To me, YSL Opium is Vanilla and spices with just a hint of patchouli. Possibly Snake Oil or one of the Snake Pit scents would be close. I haven't smelled YSL Opium for awhile, but my tormented memory says that Cottonmouth may be similar. If I find some Opium and it smells like anything I know of, I'll post back here.
In the meantime, you might just look for things with notes in common. Hopefully someone else here has Opium and can give you a solid answer. :-)
MrsV
ETA: After some searching through the reviews part of this forum, the following are BPAL scents that people have specifically said smell like YSL Opium to them:
Anathema (Sin & Salvation)
Darkness (Ars Moriendi)
Scherezade (Bewitching Brews)
Maybe you could get some imps of those to test out if no one else responds here.
Good luck!
MV
This post has been edited by MrsVeteran: May 25 2007, 11:29 PM