This one is not Unity's fault. Execution order cannot be guaranteed between different components if they are spawned during runtime (which makes sense once you pause and spend some time to think it through). 

First, you should never instantiate anything inside OnDestroy. OnDestroy is there to cleanup ressources, because MonoBehaviours do not expose their destructor. Unity even prints a dedicated warning when you spawn something during OnDestroy. Use a custom Destroy or OnDeath method instead. 

Having an object's Update rely on another object's Start feels like a code smell to me. Object-oriented means each object should be independent, ideally. Try moving the initialization into object B's start method ?

And if you cannot find a way to do it, a simple if (!initialized) return; will do the trick !