Hello!
The OS line of engines are first class and though MVVS make good engines too they do not reach OS high level of manufacturing refinement and easy handling qualities that is the trademak of every OS engine. The MVVS is also very easy to adjust but the 15cc engine does have a little more tuchy idle needle. You have to losen a screw and then turn a small exenterscrew and then set the first screw again. A little more cumbersome than on the OS.
But after you set the needles on the MVVS itīs a very reliable performer capeable of slightly lower idle rpm than the OS.
It also performs perfectly on 0-5% nitro.

For a modeler with a few years experience though I would say itīs a toss ............, regarding starting , throttle adjusting, idle adjusting and all out handling qualities....but for a total novice ...The OS has the edge....But on the other hand.....a total novice should not get such a big engine in the first place.
The MVVS is quieter though than the OS and you have the possibillity of changing the muffler to a minipipe for added power.
I would also think that the MVVS is a lot cheaper.

Regards!
Jan K
Sweden