Have you had a chance to see the ring in person? To me, the main characteristic differentiating OMC from OEC is symmetry: OEC's were cut with more sophisticated equipment, and are more symmetrical than OMC's. If you have the chance to see the stones in person, it's fairly easy to check the symmetry of the table and the larger facets. Are the 8 sides of the table fairly even? Are the larger facets pretty much the same shape?
I'm leaning towards OMC, BUT the picture does not show detail that clearly. A better picture might change my mind.