O
- Olympia delle Tofane
- Olympiapark Loop
- Olympia to Pe Ell
- Olympic
- Olympic Hot Springs Trail
- O'Malley Peak: Ballpark Trail
- Omalo to Girevi... the Adventure begins!
- Omas and Opas (aka The Twins)
- Ombrieux line
- Once is Enough
- Once is Enough
- One Mile Lake Loop
- One Time Only
- Onion Creek Trail
- Onion Creek Trail
- Onion Creek Trail
- Onion Portage
- Onkeljoch
- On the front line: Lagazuoi and Forcella Travenanzes
- Onza Alley
- Oolah Valley: Anaktuvuk Pass to Summit Lake
- Oolah Valley: Summit Lake to Dalton Highway
- OO Left
- Oompa Loompa Land
- Oops and Poops
- OO Right
- Opabin Prospect Out-and-Back
- Opal Hills Loop
- Opal to Grizzly Traverse
- Open face - Past trees & turn right from top of Gron
- Oracle State Park: Big Loop
- Oracle State Park: Nature Loop Trail
- Orange Loop
- Orcs
- Oregon Trail Reserve: Rim Trail Loop
- Orient Express Couloir
- Orient Rope Line
- Original Sin
- Ornach Ski-tour
- Orsières, Champex, Chamonix 57km
- Orsières - Moay with the Escargogo
- Ortler Ski Tour: Branca Hut to Pizzini Hut
- Ortler Ski Tour: Martello Hut to Branca Hut
- Ortler Ski Tour: Northeast Couloir of the Gran Zebru from the Pizzini Hut
- Ortler Ski Tour: Pizzo Tresero from the Branca Hut
- Ortler Ski Tour: Punta San Matteo from the Branca Hut
- Ortler Ski Tour: Sulden to Zufal Hut
- Ortler Ski Tour: Zufal to Martello Hut
- Ortveri (4365 m) NE Face
- O-Sawa
- Osberg Ridgeline Epic
- Osorno via La Picada - North Face
- Osorno Volcano South West Face
- Ossepi Chute
- Ossmann Alm Trail
- Ostrinne
- Ostuferwanderweg
- Ostuferwanderweg am Hallstättersee
- Ost Wand
- Otter Slide
- Otterstrom's Log Park
- Otztal Ski Tour: Hinter Schwarzenjoch from the Martin Busch Hut
- Otztal Ski Tour: Hochjoch Hospice to Vernagt Hut via Fluchtkogel Summit
- Otztal Ski Tour: Langtaufererspitze, 3527m
- Otztal Ski Tour: Martin Busch Hut to Hochjoch Hospice via Similaun Summit
- Otztal Ski Tour: Vent to the Martin Busch Hut
- Otztal Ski Tour: Vernagt Hut to Vent via the Wildspitze Summit
- Ouachita National Recreation Trail
- Ouachita National Recreation Trail
- Ouanoukrim
- Ouita Coal Trails
- Ousel Falls
- Outer Limits
- Outer Limits Traverse
- Outer Mountain Loop
- Outer Planets
- Outland Loop
- Outlaw Klettersteig - Kleiner Krippenstein
- Out of Control
- Outta Sight Outta Mind