|I think that the OS really do a good job of fitting most kids until PTing. I used the OS on my first son until he PTed around 34 lbs, about 26 months or so. I could still use the OS with a liner, although they fit best if I used them on the their last snaps (i.e. did not have two snaps done on each side, just one). If no liner, as was usually the case, then they were okay to do both snaps on each side. He was pretty average for height, but very chunky. ME does make extender things for the snaps in case you get close but not quite. My second son is now (fingers crossed!) PTed at nearly 3 years, but only 28 lbs, and he has plenty of room yet in the OS.
From my time on this forum, it's usually on the newborn side of things that people don't like the OS (if they don't like them), because they can be a bit bulky when they are folded over for a newborn. Once they are on longer folded, they're quite a trim fit, and although it can really depend on baby's build, my impression is that most people really are able to use them to the end. The exception to that might be for a very big child and/or one who is very late to PT (or perhaps just needs a night diaper for quite a while, but that's very hard to predict, so I figure not really worth planning around).
All of that being said, the L Sandys do tend to fit a bit more generously than the OS, even though they have the same upper weight rating, so if your child is already pretty big for her age, that might be a consideration. They are also more absorbant (and thus a bit bulkier), so I've liked them for night use.