I've been asked a time or three, so I'll say there's two main ways to pronounce Oneirotheria (and any others you want aren't wrong).
1. I, with my accent, say it oh-NAIR-oh-THAIR-ee-yah.
2. A more etymological pronunciation might be oh-NEER-oh-THEER-ee-ah (with a soft and breathy TH-).
But as you will, she will respond if she wants however you say her name. And be silent just as oft.
The Witch & The City