The oval mouth double jointed snaffle is basically a variant of the French link, with a rounded lozenge instead of a flat spatula joining the two halves of the bit. The benefits are largely the same, with reduced nutcracker effect, more even pressure over the bars, and independent control over the two sides of the mouth. Some horses prefer the rounded lozenge as it perhaps encourages them to mouth the bit and obviates any thin edges that could be uncomfortable.
The lozenges are typically designed so that the eye hole of the jointed arms is open when looking at the bit straight on. This is different from the French link, in which the eyes on the spatula are in that position and the eyes of the arms are at a 90 degree angle. The former type of jointing would seem to be preferable, in that it would be less bulging against the tongue, but in practice, it probably comes down to individual preference.