One of the nice advantages of harnesses with o-rings is that you can switch out the rings for larger/smaller dildos. Depending on the specific harness, you might still be constrained by the cutout in the back pad of it, but usually that permanent hole will be large enough to accommodate most dildos (within reason).
Also, I'd recommend getting some metal o-rings if you do plan to use multiple sizes- rubber ones tend to stretch too easily.