Stuart, you can TAKE the Nitrox course concurrently with the OW course, but the Nitrox card will not be issued until the OW cert is issued.
PADI is not the only agency in the world. Or even in the U.S..
As I said, I only read the standards for SDI. Their standards say the cert will be issued when the student has met the requirements of the course, and the requirements don't say anything about finishing OW.
But, out of curiosity, how would it be handled if a student wants to take Nitrox before they leave to go somewhere and do referral dives to finish their OW?