Four strokes were around long before OS but it was rare to ever see one. OS just got out the first good production model. Between the Saitos and OS engines I haven't seen a nickles worth of difference as far as the power or quality and right now the prices are about the same. I like pumps on my engines so that leaves Saito out completely, they have no pumped engines. Price for price I have gone over to YS engines several years ago so that leaves out Saito and OS.