Welcome to Smogon! Take a moment to read the Introduction to Smogon for a run-down on everything Smogon, and make sure you take some time to read the global rules.
The OP format I think is the best but I think an AG slot or really anything else could go in place of the BO3 slot, I personally feel that there's enough Natdex OU. Other than that good looks
3 Nat Dex Ou slots are more than enough imo. You can easily use th b03 slot for a nat dex om. I don't play Nat Dex Oms but i feel they should get due representation.