Keith is correct. The Opel's 12 mm x 1.5 pitch thread is common these days. Almost all European and Japanese auto makes, plus quite a few US manufacturers, use this thread. A notable exception is Subaru and Nissan, which use the same diameter but different pitch thread....don't try to make them fit your Opel!!
I fond a set of lugnuts that are 12mmx1.5 but they are bigger than the standard(19mm) lugnuts.The bevel looks the same but the tapered end dosen't fit in the lugnut holes in the rim as far as the stock.will these be ok or do I need to find a closer match?the set of 16 plus 4 locking nuts and tool to remove them was $20.00.
I think even my Chrysler mini-van uses the same thread and pitch, but uses a 21 mm head. I have seen this type used on Opels (Katheryn McCoy's GT to be precise, until I changed them out for the right nuts) and they seemed to work. But it's not so good to only use one (or three) on a wheel, since they weigh quite a bit more than the 19 mm nut, and might cause a balance issue. So long as they have the right bevel, they should work. But if the bevel is wrong, DON'T use them, since an Opel wheel is "lug-centric" (centered by the lug nut) versus some makes that are "hub-centric" (centered by the center hub).