Located in the north of the Tantalus Range, Ossa Mountain differs markedly from its close neighbour Pelion Mountain in that its north face is to alpine climbing what Pelion's is to skiing. The rock is predominantly granodiorite and well suited to climbing. Its steep north faces remain cold throughout the year - ice can be encountered most of the year. The main summit is guarded by a notch on both the east and west sides. The western notch can be scrambled through and the eastern notch is negotiated with a rappel and/or low-5th climbing. The first ascent party called the eastern subsummit on the far side of the notch the "East Peak". The highest point on the main summit is very close to the East Peak and the east notch. Normal approach is via Sigurd Creek Trail although the original ascent in 1960 was from Zenith-Tantalus col.
The northeast face is a steep glacier that is protected in late summer by an increasingly difficult bergschrund. It is also likely that its east ridge was climbed on the first ascent of Ossa Mountain because the party negotiated the gap between the eastern and main summits. A sinuous north ridge also connects to the East Peak and was climbed in 2001.
The west ridge is the standard scramble and descent route. A prominent north-west buttress can also be used to access the ridge, with varying degrees of difficulty depending on the route.
There are no known climbing routes on the south face of the mountain. Unfortunately, due to its angle and intervening ridges, no one except Clowhom River loggers has ever seen it. The only feasible approaches are via vertical snot in Clowhom River or dropping down from Ossa's W ridge or Pelion's south ridge. Clearly, this is an ultimately masochistic endeavour for half bush apes, half climbers. There is a shallow glacier in a deep hole at the base of the south face with an equally uninviting lake with permanent icebergs.
All first ascents and variations were accomplished by hiking all the way in from the Squamish River.
Name Notes: From Neal Carter's "Early Climbs in the Tantalus Range": Legendary Mt. Ossa in Thessaly is the one upon whose summit the giants piled Mt. Pelion in an effort to attain the summit of Mt. Olympus, home of the Greek gods.