|Where the OS will fall on baby's belly is a function of baby's build and what setting you're on. At 5.5 months, he might still use the folded-over setting, so then they are likely to fall at the navel or below. There will be a period later on when you stop folding over the front of the diaper that they may rise a little higher, but this would be temporary, as baby will keep growing. I would say that in general the diapers have tended to be at navel or below on my boys.
They tend to fall (or work themselves) below that belly bulge anyway.
That being said, I'm not sure that on my boys the OS have been lower than disposables (though I haven't used disposables at many points). Because there is a separate cover, though, neither the cover nor the diaper has to be very tight, so perhaps you will be less worried about where the diaper falls because you'll be able to feel how much flex there is in the fabric and elastic on the cover. With the diapers especially there isn't a tight line of elastic like there would be on underwear, so even a position that you might find uncomfortable likely won't be on baby. If baby isn't moving much, it won't matter much where the diaper falls, and once baby starts sitting and moving more, the diaper will probably work down under where baby bends, anyway.