Actually the surf is mostly bigger on the North Shore of Oahu then in Waikiki mostly  in the winter months.  More specifically when the winter is in the Northern hemisphere.  During this time there is a lot of storm activity in the North Pacific which turns up the ocean and sends swells across the pacific.  Some of those swells reach the North Shore of Oahu and create waves. Fortunately for our surf school and our surf students there is a protected cove near Haleiwa town on the North Shore where the waves are always small and safe.