very sure DD is CM11S. note that sometimes DD ppl blur... don't even know the difference between CM11 and CM11s. when i bought mine, the first sales fella who attended to be didn't even know what I was talking about when I asked if its ColorOS or CM11S.

bestsane if really it comes with CM11 (which i don't think so), then ya, just flash CM11s and you should get the OTA after that. in any case, if you are willing to try flashing, i do suggest you do that even if CM11S is already installed -- just to be safe coz you never know where they got the CM11S from. otherwise, CM11 will not get OTA that is for CM11S. think of CM11 and CM11S as different roms.

plus, OnePlus One has got many official ROMS. who knows, you may actually find something else that you like instead of CM11S.