O.S engines are made in Japan and have been on the market for years. O.S are generally regarded as great engines, they are of a high quality and run fairly well. They are used by racing people in 2.5cc rat racers and by aerobatics flyers using 6cc sizes. OS are more expensive than MVVS but are more of a combat/aerobatic engine than a racing engine, but they will work well in some of the more slower classes of racing.