Nikon D800 or Oly OM-D?

Yes. The OM-D e-m5 is as good or better than entry-level or middle-of-the line DSLRs by Nikon but can't compete with top of the line cameras from that company (like the D800).

1. Nice bokeh : it all depends on the lens that your buying. Just look at the many examples of photos in this newsgroup and you'll find very good ones (the M.Zuiko 60mm for example)

2. Low noise : Judge by yourself (at ISO 6400):

3. IBIS noise : In a perfectly noisless room, maybe...

4. Lens for taking baby pics : Yes the Lumix 12-35mm would be a good choice (that's my favorite lens). Or you might give a look at our primes : Leica DG 25mm F1.4, the Lumix 20mm, maybe the M.Zuiko 17mm.