I bought a knockoff from eBay just before Christmas. It took s month to arrive.
The tone was delicious and sustained -- a match for the Oi! Recording.
The problem? Unlike the Knog Oi!, the clamp would only fit a 22.2mm diameter and had a web molded over the portion the Oi! Uses to route cables. It sat in the cold (-11°C) garage and cracked when I tried to expand it. Not entirely surprising.
I think if I had left it alone or unsocketed the quill stem I was trying to fit it to, all would have been fine. Cost: USD $1.28 including shipping, little enough to give a try.
I my have another go, this time using my Dremel to remove the web from the clamp and work on it inside the house, where it is warmer.
Best,
Dan.