Oahu's fame resides mostly in Waikiki and world-class surf breaks, but its hikes deserve equal credit. In prehistoric times a shield volcano on east Oahu split apart and slid into the ocean. What remains is the Ko'olau Range, a jagged wall dividing the south and windward shores. Ko'olau ridges pleat the south shore and offer an array of jungly hikes for all abilities. Here are my favorites in order from easy to difficult.
Manoa Falls, 1.5 miles
The road snaking through the Manoa Valley neighborhood metamorphoses into dense rain forest before arriving at the trailhead. Chickens cross the road and the smell of red dirt permeates the air. Thus the lovely paradox of southshore hikes is illuminated: trekking through jungle that lies just a few minutes beyond traffic, noise, and civilization.
Manoa Falls is quite popular, so I suggest going on a weekday morning rather than a weekend. It is easy for most and takes about 45 minutes round trip. The trail follows Manoa Stream all the way to the waterfall. Some guide books show people swimming in the basin, but I do not recommend that. From the waterfall, hikers can choose to commence the bamboo forest trail to the left, adding on a few more miles, or back track to the trailhead.
Koko Head, .24 miles
1,143 railroad ties, and you're done! This straight shot up Koko Crater delivers huge rewards without sacrificing a lot of time. The path follows an old military railroad track and finishes at some old bunkers at the very top. Locals hike or run here often for their daily workouts. It offers views of Honolulu, Hanauma Bay, Sandy Beach, and even Molokai and Maui on a clear day. This steep climb takes most people 30-45 minutes one way. I highly recommend it for those who want a quick workout, or want to squeeze in a good hike before eating at Kona Brewing Company or snorkeling at Hanauma.
Kuli'ou'ou, 2.5 miles
This intermediate hike passes through so many ecosystems you'll want a biology teacher by your side. Allow 2.5 to 3 hours for this hike. This trail begins in the Kuliouou subdivision and switchbacks through piney forest to the ridge. From here, the trail wanders up and down along the ridge to the sparsely vegetated Ko'olau summit. You'll want to rest here for a few minutes to take in the great windward views.
Palolo Valley, 3.9 miles
This hike follows Wai'oma'o Stream up three waterfalls in Palolo Valley. Intermediate to difficult, this is not for people afraid of climbing ropes and falling slipping down waterfalls. The first big view awaits at Ka'au crater, a lush bowl tucked into the Ko'olau range. The trail follows the crater rim up to the Ko'olau summit, unleashing spectacular views of the windward coast.
Mount Olympus, 6 miles
This intermediate trail starts at the top of St. Louis Heights and follows Wa'ahila Ridge, the eastern ridge of Manoa Valley. The trail starts in ironwood forest and meanders through common plants such as Christmas berry, silk oak trees, and the native alahe'e. As the altitude rises, strawberry guava, 'ohi'a and koa trees are abundant. Views of Diamond Head Crater and Honolulu get better with every step. Eventually the trail grips the ridge, and those with fear of heights should turn around at this point. Some of the climbs are steep and eroded. They can be quite scary on windy or wet days. The first summit gives a sneak peak over the Ko'olau Range. Two summits later, and you are at Mount Olympus with sweeping views.
Lanipo, 7 miles
The advanced hiking trail follows the ridge with Palolo Valley on the left and Wai'alae Nui Gulch on the right. After climbing over a few rock outcroppings, the trail descends into native koa tree forests and later through ironwood trees. Hikers should stop every now and then to photograph unique views of Koko Head, Maunalua Bay, and Diamond Head. The trail ascends further into dense rainforest. Numerous ups and downs lead past Ka'au Crater and finally to my favorite summit on the south shore. The panorama is a mixed plate of some of the island's most famous landmarks: Olomana, Kaneohe Bay, Kailua, Makapu'u, and even Chinaman's Hat. Do not attempt this trail right after or during rain.
For more detail on these hikes and more, I highly recommend the book The Hiker's Guide to O'ahu by Stuart M. Ball.