I had an OS 28xz in my truggy and it was a pig of a thing.
I had to replace the carby straight up, it was faulty from the factory. It had a lot of slop in the rod at 1 and a half gallons (6 litres). Would not hold a tune.
In the end the crank shaft snapped at the flywheel. That was the last straw, I was not throwing any more money at it.

I know some people swear by them and dont have trouble. I dont know if I got one from a bad batch but there were a few guys having trouble at my club with the 1st batch of engines.
It has put me off OS engines for life.
MassiveMods is right about the OS rebuild costs, I think they are just too expensive and not robust enough.
Where the internals of a GO look thick and strong.

I put a .25 Go in and a 0801 pipe and love it.
Great fuel economy plenty of power to push a heavy Hyper ST Pro around the track.
I have had 2 and a half gallons (10 litres) of trouble free reliable running.
Can easily get 8 mins run time in a truggy, never tried but probably could get close to 9.5 mins with a smaller carb insert.