I'm going to sound like a real hypocrite now.....
I first fitted an Ox spring to a Webley Vulcan many years ago. The gun cocked like a pig and shot like a pig. After a few hundred shots the holes at the back of the action had elongated.
I fitted another one (again many years ago) to a HW77. I nearly lost an eye fitting the damn thing. The gun again cocked like a pig and shot like a pig. The recoil made you think it must be doing @ 50FP but it was actually doing 13FP....still over the limit.....the spring was removed after just a few shots. When I removed the spring the rear spring guide was totally ruined. The spring had ripped it to bits.
Now the hypocrite bit.....some time later I bought an old Diana 38 off a local old guy who had quite a collection. It was .22. The gun seemed to cock and shoot well enough. I tested the gun and it was doing 14FP. I stripped the gun and inside was a square section spring......I'm presuming an Ox. However on this spring the square coils seemed to sit squarer (I know that sounds daft). In other Ox springs that I've seen/used the coils sort of bend over so the spring is presenting a sharp edge in contact with the guide etc. Maybe this had been worn in? Anyway I replaced it with a round spring but kept it.
Just last year a local lad brought a HW77 to me that he said wasn't shooting right. I stripped the gun and the spring was broke. He hadn't got any money so I told him I had an old square spring that I could fit (it was the right size) but couldn't promise it wouldn't wreck the gun. The gun looked like it had been mis-used anyway. He said to fit the spring. I fitted the square spring with a top hat. It fitted the rear guide really well and the length was probably no more than a Venom F1.....as I say this spring may have been well worn in the Diana.
The gun cocked and shot as smooth as anything I'd owned. It shot .22 at @ 11.5FP and was very very consistent. He's still got it now.
As I said....I'm not sure if this was an Ox but it worked well.
I'd still say don't buy one.
Boz