Originally Posted by xliioper

The Delta T&C's page is linked a couple of posts above. For DL marketed and operated flights, there is no longer MUA availability for flights within North and South America region. As for AM operated flights, OY inventory is almost certainly only applicable to DL flights and I would expect there to be some other AM bucket for AM availability. Here is what T&C's say about AM flights if you go to them --

• For Delta-marketed flights operated by Aeromexico, Mileage Upgrade Awards are available on Delta’s published fare classes in Y, B, M, H, Q, K, L, U or T for travel between the United States and Mexico.