Which mountain range is located to the east on Oahu?

The Koolau Mountains are indeed located to the east of Oahu. This mountain range is a significant geological feature of the island and plays a crucial role in the island's ecology and climate. The Koolau Mountains are characterized by steep cliffs and lush valleys, which are the result of extensive erosion from rain and wind over millions of years. These mountains also contribute to the distinct weather patterns on Oahu, particularly in terms of rainfall, which is generally heavier on the windward side, where the Koolau range is situated.

In contrast, the Waianae Mountains are found on the western side of Oahu and therefore are not associated with the eastern part of the island. Mauna Kea is not located on Oahu but is instead situated on the Big Island of Hawaii, making it irrelevant to this question. Diamond Head, although a notable crater and landmark, is also found on the southern coast of Oahu, not exclusively on the eastern side. The Koolau Mountains uniquely define the eastern landscape of Oahu, which is why this choice stands out as the correct answer.