Opinions vary, since there's no definitive pronunciation available.

Personally, I pronounce it "Öy-ill-vert".
"Ö" represents a kind of "er" sound, but without the "r" at the very end. In German, "Ö" and "Oe" are almost interchangeable; I used that as the basis for my pronunciation.

But I guess you can't go wrong with "Oy-ell-vert", either. The "Oeil" part sounds a lot like "oil" when you pronounce it like that, but not quite. I figure that every letter is significant in a word like that, so I put a little more emphasis on the e.