Do you include the clinically negative contralateral neck in your field for a lateralized oral cavity who has N2b disease after LND?  

There is early N2b and advanced N2b. Generally, no. Lots of disease and ENE, now N3, where obstructed lymphatics could result in crossover, then yes. Medial extension of the primary into the tongue base (unlikely) would also lead to contralateral neck RT. Dose would be 50 Gy in 25 fractions or equiv...

Agree with Dr. @William M. Mendenhall. In addition to the risk of contralateral lymphatic diversion in N3, similar risk exists in patients who had previous neck dissection; it takes few months to the lymphatics to divert through the submental space contralaterally after neck dissection. In that case...