The island of Oahu, like the other Hawaii islands were formed due to hot spots. Hot spots are areas on the Earth’s surface where volcanism occurs. Usually this will be around the mid-ocean ridges or in some cases, in the middle of tectonic plates. The creation of the Hawaiian Islands started million of years ago when lava started to push up through the ocean floor. Over time, this lava cooled and accumulate to become an island.
Now Oahu was formed in the same manner, but was created from the combination of three volcanoes. It has been thought that the two that made up Oahu were the Waianae and Koolau Volcanoes around 3 million years ago. New evidence shows that the two actually formed on top of an older volcano.
In fact, the rocks from the north west area of the island were always thought to come from the Waianae volcano. The under water ridges by this section has shown that the rocks are in fact, 1 million years older than the Waianae ones. So this means they used to be part of a volcano that had at one point a 1000 meter peak sticking out of the water. This lost volcano was named Kaena and thought to have cooled off around 5 million years ago. This receding peak had the other two erupting volcanos add to it, which over time created Oahu.
The Hawaii Diamond Head Crater was formed from the Honolulu Volcanic Series, about 500,000 to 150,000 years ago. Other famous sections on the island were formed during this time from quick eruptions like the Koko Head Crater and the Punchbowl Crater. Scientists think that Diamond Head is monogenetic, where it only had one eruption and thus will never erupt again.