I am in the process of applying to NCP and am also prior service. Here is my understanding of the process. Once accepted, you can be commissioned the day that you graduate. BUT you do not start your active service until after you pass the NCLEX. So when you leave for ODS, you are a official active duty, commissioned officer.
PalmHarborMom is correct, nurses are direct commission officers which means you are commissioned prior to leaving for ODS. When I was signing my papers for the NCP and enlisting there was a recently graduated NCP student that was being commissioned. He had already passed the NCLEX and was leaving for ODS in about a month.
You don't receive a commissioning until after you have passed the NCLEX. You do not start getting paid as an active duty service member until your report to ODS. You do not get official orders to your duty station and ODS until a week or two after passing the NCLEX.
Source: the NCP website, and the NCP program manager.