Where I fly, OS is most common in two and four stroke. Saito or Magnum would be number 2 in four strokes.

After the flood of OS in two strokes, I see (in no particular order) TT, Tower, Webra, GMS, Evolution, Magnum

My two stroke engines are
TT 61 Pro, ASP 61, OS 25 FX
My four strokes
OS 70 Surpass, OS 91 Surpass II, Magnum 91, Magnum 70