Does anyone specifically know the exact differences between an Olds 12 bolt and a Chevrolet 12 bolt? Will the internals/axles etc. swap?

The internals of a 64-72 BOP and Chevy 12 bolt axles WILL NOT interchange.

A Chevy rear-end uses "C-Clips" (retainers) to hold the axles in the center of the carrier. A BOP uses Bolt-in axle retainers at the wheel-end of the axle.

There are 12 bolt (covers) BOP rear ends which only have 10 bolts that bolt the ring gear to the carrier. You have to remove the cover to confirm 12 bolts in the ring gear, that's where the name actually comes from for Chevy 12 bolt rear-ends, not the outside cover.

So, no I do not have 1st-hand knowledge of other EXACT differences. But I have seen the ones above, and when you check parts online you will also see this criteria.

Kewl, thanks for the info DG.... but if I had a complete 69 Cuttlass 442 12 bolt, I could install a 12 bolt Detroit True-Lock posi unit in it and then swap the complete Olds rear into a Chevelle, correct?

BOP is actually a 10 bolt, different ring gear size, and last time i checked only one company made a new posi for them and it was at least $450
-ryan

The Olds will have the "O" axle, which has 12 bolts in the cover, but only 10 in the ring gear. The ring gear is 8.5 inches, where the 12 bolt Chevy ring gear is 8 7/8 inches. You can not put 12 bolt Chevy parts in the "O" axle. The axle splines are also different. The "O" axle assembly will bolt into the Chevelle. There are new Traction Lock differentials and several popular ring & pinion sets available for the "O" axle.

Is the Old "O" Rear as strong as the Chevy 12 bolt?

no, but it is stronger than the GM 8.2" 10 bolt and it doesn't have the c-clips which is nice. It's about as good as a 8.5" 10 coroporate 10 bolt with c-clip eliminators, but you can't get parts as cheap and easy.

That's right. The parts are quite a bit more expensive. About the same strength as the 8.5 corporate rear end.