Then you would be in standard...
Many 12-string guitarists tune down a half-step or even a whole-step below standard.
1. easier to fret
2. IMO, you get a bigger sound out of it
3 Most importantlym I think...reduces strain on the neck and body. I would wager the majority of older O 12's that have been kept at standard, have the neck caving into the hole, to some degree. If you want longevity out of your O12, my suggestion is to tune down.
If you are buying a used O 12, or any 12 for that matter... Check that neck angle!!!