From: Santa Rosa, CA
The key is flying naval interdiction just off the port during both friendly and enemy player turns. Ideally use the Ostsee formation as these naval only units are good at adding naval interdiction. If you generate control of sea hexes next to Odessa you can isolate the units in the port. But you must do it in both friendly and enemy air phases. Once you've isolated the port, even if the enemy depot in Odessa has freight, there are some negatives to the isolated units that will make it easier to storm. The penalties are not as big as in WitE, so if it is a strong garrison, you will still need a good sized attacking force. Taking with Romanians only is almost impossible. As the Soviets pulled out of Odessa in October, it's arguable that it would not have fallen without more Germans troops involved. If you isolate the port, and then bomb the railyard in the hex (maybe also port, not sure), to destroy the freight there, you can deplete the depot and eventually run the defender out of supplies and ammo. So you can try for a long siege if you don't want to use 11th Army to storm the city.
All understanding comes after the fact.
-- Soren Kierkegaard