I have several of them: OS.15FP - stout, dependable power AND very quiet. Older OS MAX 15 - Not sure what it's designation is but it's the one that has a baffled piston (loop scavenged) with strap on muffler. Strong runner, a little less power and somewhat noisier than the FP. It has an airbleed carb but idles well. Great little engines!
Matter of fact, I've accumulated and stored 2 extra of each of both types that will someday be on twin-engine aircraft.