The 'O' stands for the circular nature of this advanced, multi-day hike (called 'Shengleng Trail' on the national park website), which begins and ends at Wuling Forest Recreation Area. It may also describe the shape of your mouth upon seeing some of the best high-mountain scenery in the country. The trailhead is on the path to Taoshan Waterfall.
This is not a trail for the faint-hearted or the inexperienced. You will be required to scramble up and down scree slopes, navigate narrow ledges with 1000m drops on either side, and use fixed ropes to climb vertical shale cliff faces. It's a grand adventure, but you need to be prepared.
After a hard push on the first day to the ridgeline, you never drop below 3000m for the next four days as you reach the summit of a half-dozen peaks including grassy Chryou Shan (池有山; 3303m), crumbly Pintian Shan (品田山; 3524m), black-faced Sumida Shan (素密達山; 3517m), sublime Snow Mountain North Peak (雪山北峰; 3703m), and Snow Mountain Main Peak (雪山主峰; 3886m) on the last day. For a full description of the five-day hike see hikingintaiwan.blogspot.com.
The Holy Ridge (without the O) has several variations. The most popular is a linear path going from Dabajianshan to Snow Mountain.