from Webster’s unabridged third new international dictionary:
Oneir- or oneiro- also onir- or oniro- comb form [Gk oneir- oneiro-, fr oneiros, oneiron; akin to Arm anurj dream] :dream <oneirology>
I believe it is pronounced Oh-Nay-Ros, though I may be mistaken. As for the two, it helps it to sound more like a science-fiction planet. (Yavin IV, Theban II, etc) It’s also there for another reason, but I can’t tell you right now 
Sorry.