I own or have owned both the Saito .82 and the OS .81-FSa.

The OS is the better of the two engines in my opinion. That's not saying that the Saito is a bad engine, because it isn't. The OS is just a newer design that works a little better. It seems to make more power although that is just anecdotal evidence based on pulling the same prop on the same fuel; I haven't actually measured it. The nicest thing though is that the OS .81 spits very little oil out due to the lubrication system design. It is even cleaner than the older OS engines.

When most people compare OS and Saito 4-strokes they are thinking about the older OS's, not the newer FSa line. The FSa's are outstanding. I own both the 1.10 and the .81.

You really can't go horribly wrong either way of course. It comes down to preference and what you are willing to spend. I'd go for the OS though; that's just me.