Which OS35 is it? If it's got a cast iron piston and you're using normal RC fuel then you're lucky the engine won't keep running
If the compression feels OK then you've either got a bad plug, bad fuel, or the mixture is way out. Is it cutting out at idle or full throttle?
Just a thought, but if it's the older baffled piston type, is the piston in the right way round? Is the liner in the right way?