Hi jsj5g,
I have more information about the OSO Mattress in the One-Size-Fits-All thread here, and I copied some of what I wrote for you below. This mattress is from Reverie, who is a member here, which means I think highly of them.
The OSO Mattress contains a 1.5" layer of blended Talalay latex on top of a 1.5" layer of 2.0 lb polyfoam as a transition layer and a 6" 2.0 lb polyfoam base layer. The base layer has a cutout under the shoulders and upper torso that contains 3" Reverie dreamcell cylinders which are made from 70% natural/30% synthetic latex which creates a softer zone under the shoulders and upper torso which can be beneficial for side sleepers especially because the shoulders are generally wider and lighter than the hips and having a softer layer under the shoulders allows the shoulders to sink in more deeply which can improve pressure relief under the shoulders and also can also improve spinal alignment in the upper body. For those that either need or prefer more firmness under the shoulders and upper body then you can also rotate the mattress 180 degrees and then the softer zone would fall underneath the lower legs and calf where it will have no effect on the shoulders or upper body alignment and would be similar to a single zone mattress. These are all good quality and durable materials and there are no weak links in the mattress relative to any weight range.
Phoenix