|Congrats on the baby and on exploring the wonderful world of cloth!
The difference between the OS and the OS dry is that the OS dry has a stay-dry barrier sewn in on the interior part of the diaper. This keeps baby a little drier and in theory can make poop cleanup a little easier because the surface is smooth rather than the loops of the terry.
Personally, I have opted for the regular fabric, and I then add a homemade fleece liner, which does basically the same thing as the stay-dry, but is lots cheaper, since you just buy cheap fleece and cut a liner shape, no sewing needed. ME does also sell separate stay-dry lay-in liners, but again, personally I'd save the money and go with fleece.
One advantage of using fleece is that when you are trying to potty train and want your child to feel more wet, you can stop adding the fleece.
There are no wrong choices with ME. I will respectfully disagree with the PP that most people use XS Sandys at first. The XS Sandys should give you a trimmer fit on a newborn then the OS or S Sandys, for certain, but if you can tolerate a little bulk, the OS or S Sandys should be just fine from birth. I would guess that the majority of ME users actually don't use the XS Sandys, just based on conversations on this board, but it sounds like most who do like the way they fit and perform. They also do have good resale value, and if you are interested in using XS, you might consider saving money by buying used.
The listed weight for the OS and S Sandys is 8 lbs, but I have used both successfully (i.e. no leaking!) from 6.5 lbs on my scrawny premie (and I honestly didn't think they looked that bulky on him). Bulk perception can really depend so much on baby's build (I did find cloth bulky at first on my mongo DS#1) and your own personal preference and expectations.
You only asked about the OS types, but if you are curious or considering the Sandys, try doing a search on something like One Size Vs Sandys to get people's opinions about the two styles; they are both great diapers and people often just prefer one or the other. Cost and number of children currently in diapers can also weigh in to the decision.
As PP said, you will need a cover (these come in sizes by weight) for all of the ME diapers except their All-in-one.