Hi levander,
OMF is one of the better quality and value regional manufacturers and best of all they are completely transparent about the quality of the materials in their mattresses which is one of the most important parts of shopping for a mattress. They also use higher quality materials in their lineup than most larger brands.
You can see a few examples of the construction of their mattresses in post #32 here. Some of their pillowtops use some very high quality HR polyfoam which can be a very durable and long lasting material that approaches latex in some of its performance specs.
The best way to buy a mattress / topper combination is to carefully and objectively test the specific combination in person for what I call PPP (Pressure relief, Posture and alignment, and Personal preferences). If you can test the two together it can be a very flexible design for a sleeping system because you allows you to choose the quality and performance of both the support layers and your comfort layers individually and also allows you to replace the topper if it softens or breaks down faster than the base mattress (which will often or even usually be the case) without having to replace the whole mattress.
Without testing the combination itself though … you would be introducing a variable that would have some risk attached to it in terms of making the best topper choice. The “best” topper will also depend on the specifics of the mattress you are using it on and particularly the details of the top layers in the mattress and how they interact. All the layers of a sleeping system will affect every other layer to different degrees. In these cases (where you haven’t tested the specific combination) … the best option would be to sleep on the mattress itself until it had broken in and you had completed any adjustment period to a new sleeping surface and then choose a topper based on the topper guidelines in post #2 here and the posts it links to so you can use your own personal experience to choose the best topper.
In general I don’t recommend buying a mattress this way where you have to deal with several unknowns or variables unless you are forced to because of a situation like a comfort exchange where good quality and value is not available in the store you are forced to deal with in which case it’s generally best to buy a mattress which minimizes the use of lower quality/density polyfoam in the comfort layers to the degree possible and then add your own comfort layer with a high quality topper. While this is not the ideal way to make a purchase and it’s usually more effective and better value to buy a sleeping system that you can test in person or online that doesn’t need a topper and already uses high quality materials in the first place … a least the extra risk in these “exchange” situations is offset by the even higher risks of the alternative which generally involves buying a major brand mattress that uses low quality comfort materials that will soften and break down much too quickly.
If you do decide to go in this direction … then knowing which mattress to buy that would be most suitable for adding a topper would depend on knowing the details of all the layers inside it but typically it would involve the purchase of a very firm mattress with thin comfort layers which would provide the primary support you need and then adding the thickness and softness that your testing indicated would be most suitable for your comfort layer (with a little bit of allowance for some initial foam softening). If you buy a mattress that is already close to your comfort preferences and already has some softer materials in the comfort layers … then a thinner topper would be more appropriate to take into account the part of your comfort layers that are already part of the mattress itself
Hope this helps.
Phoenix