If Russia or China does not want to waste a nuke on any of the islands, but still want to eliminate all the most important military installations in Hawaii, the environmentally friendly way is to conventionally bomb those volcanic areas on the Big Island and the resulting mega tsunami caused by the displacement of those volcano chunks that slip into the sea will do the rest. 
Simply taking out Oahu would cripple any response capability from Hawaii. Any response would be mainland based or already deployed and able to retaliate.
Oahu has the largest population, the most military resources, the most shipping everyone in Hawaii depends on, and the most airport resources.
True, there are airports on other islands, but they are ill equipped and manned to handle the overflow demand cused by the potential loss of the HNL/Hickam airports.
Not only that, but the majority of state government resides there. I don't know what kind of contingent government this state could muster should the leadership be captured or killed.
If it came down to it, a hostile nation with the right strategies and tactics could prevent shipping to the islands and even shoot down a few planes to cut off supplies. It would not take too long before the people are being starved out. We still need oil & gas to make electricity and operate most cars, buses, trucks, and heavy machinery. We also still need to obtain medicines and other critical products just to sustain life.
Once the supplies on hand run out, if we can't rely on commercial or government transports to supply us, we're basically in the position of self-sustainment or take to the seas in small craft (Cuban boat people ring a bell?). Waiting to surrender and be taken care of by the enemy doesn't appeal to me.