I should add, you'd have to eat them for it to be poisonous, and I think there are types of hemlock that aren't poisonous at all (hemlock trees?). It just struck me when I saw your list, especially because I recently saw a forensics show that featured someone who tried to kill her husband with oleander. Incidentally, it didn't work. That said, if it were me, I wouldn't want to use the oils you mentioned until you get more information about them, particularly since they may not do a thing for hair.
http://www.ansci.cornell.edu/plants/php/plants.php?action=indiv&byname=common&keynum=63
http://www.ansci.cornell.edu/plants/php/plants.php?action=indiv&byname=common&keynum=28
http://www.ansci.cornell.edu/plants/php/plants.php?action=indiv&byname=common&keynum=59
(these people at Cornell might be good to contact for further info, by the way)
If you are looking for a good oil I have been using Moroccan Oil (argan oil). I purchased it from Dvada and it has worked wonders on my dry hair. My hair feels so soft and silky. It totally saved my hair. All you need is a little drop.
Extra virgin coconut oil is good. Other oils work too. : http://thebeautybrains.com/2007/05/14/two-natural-oils-that-make-your-hair-shiny-and-strong/
I also found this post about argan oil : http://www.futurederm.com/2008/04/13/spotlight-on-argan-oil/
The only one I have tried is the extra virgin coconut oil. I did try mixing the extra virgin olive oil and extra virgin coconut oil together, but for my hair the coconut oil on its own does just fine.
I still want to know who's telling people to put oleander, hemlock and opium in their hair. Okay, I grant you, the hemlock could refer to hemlock trees (which are not poisonous) rather than "poison hemlock"--but I'm not sure whether there really are harmless forms of the other two. I hoped I wasn't too blunt with the OP, I don't want to scare people, but it just kind of made me wonder.
Okay, so I wondered--and then I looked it up. Here goes, looks like Oleander is being recommended on the net not only for thinning hair--but also as yet another quack cure to lure people with cancer and HIV away from effective medical treatment. http://www.globalhealingcenter.com/natural-health/oleander/
I guess the opium is part of an ayurvedic remedy for hair loss http://hubpages.com/hub/Regrowth-of-hair-on-the-bald-head-is-possible-now Somewhere in my internet travels, I saw this use of opium attributed to hippocrates of hippocratic oath fame. But now remember, we've got rogaine nowadays (much less toxic, better in case of DEA raids)...
I can't find much about hemlock and hair.