Mount Ossa is definitely one of the better scrambles in the Coast Range. It's not an especially well-travelled route, and the long and adventerous approach is daunting to many people. But it is worth it for three hours of continuous good quality hands-on scrambling.
The route begins at what is effectively the end of the Squamish River Valley Road near the outflow of the Ashlu River. It's a popular camping spot because it sits on crown land, so be sure to leave your car empty of valuables just in case. Break-ins have occured. The trail starts up an active logging road that quickly turns into a abandoned logging road that has long been overgrown. From here the trail meanders through spectacular mossy rainforest. It is a treat walking through these ancient quiet forests, but the feeling fades as the trail spikes steeply uphill. You'll find a fairly busy trail here as many folks head for Crooked Falls. This popular and beautiful waterfall is short distance from the trailhead. It's a quick 100-meter detour if you fancy a break and want to see a rather spectacular waterfall. You will get many more of those ahead, though.
The trails continues ever upward, becoming steeper and less maintained. Be prepared for slippery logs and muddy paths. The trail eventually stops ascending near a large boulder, but confusion sets in. The markers become jumbled because the trail has been swalled by numerous downed trees. The trick is to head west (center right from where the trail fizzles out) and you will find the trail and markers once more. The trail heads downhill from here, but by the time it heads downhill the route becomes obvious. If you haven't found it and you start heading downhill, retrace your steps.
The going is a little easier from here on. There are lots of small ups and downs, but it's easier to keep up a decent pace. Eventually the trail pushes through some brambles and alders through an avalanche path before tucking back into the woods. These woods give way to beautiful alpine meadows with ample blueberrys in September. The trail is soft and gentle as it arks gently upward along a river. And here you get to the downed bridge. There are the remains of a bridge, but it has since been washed away, leaving two options for crossing: a risky Tyrolean traverse or wading. Wading is the more sure-fire option, and bringing a camp towel and even camp sandals makes the crossing a little easier. Trekking poles also really help. The second option is to bring a harness and attempt a Tyrolean. As tempting as it is to avoid wet feet, it's a short crossing, and the integretity of the cables that remain isn't guaranteed. Use at your own risk.
Past this there is another disheartening and steep climb before you hit a boulder field that gives way to the glacial plain beneath Pelion-Ossa. It's a sight to behold, and the view makes the hike all worth the effort. The trail markers are pretty good here as you descend toward to the toe of the glacier. Stay alert, though, as the markers disappear suddenly when they make an abrupt turn at the river forming beneath the glacier. You actually end up walking due north directly in the opposite direction. This area is gorgeous, and absolutely incredible views can be had here. Numerous waterfalls spout from many corners between the moss, rocks and trees. Being so far removed from light sources gives it a very peaceful feeling as well.
A wide open field that is often used as a camp is there. This is absolutely a good spot to stop. It does mean adding an extra 100 meters of elevation to your next day, but the campsite here is far better suited for big groups, and the view of surrounding peaks is better. It is in the Tantalus Provincial Park, however, so be warned that while camping isn't prohibited, campfires are definitely not allowed any time of year. Also, there are no nearby trees, so a bear canister is recommended for this area. It is recommended to get snowmelt for water in this area despite the temption of an entire river flowing nearby. This river is very silty, so pre-filtering with a bandana or coffee filter is a good idea. You can also leave the silt to settle, which takes about two hours.
If you want to stay at the Three Lakes site, wander to a narrow path in the alders where the orange markers re-appear and lead you through a boulder field toward steep meadows. The trail switchbacks a few times before you find yourself among the Three Lakes. It's a lovely area though flat ground that can be tricky to find, and there aren't many locations for a good bear hang. A couple of trees 100 meters out should serve. Be cognizant of where you dispose of waste because these rivers do feed streams bellow that future hikers might use for drinking water.
If you fancy a small detour hike you can follow the path up toward the Mount Ossa scramble. The first open meadow is lovely and a worth visit. Past this is an open bowl. There is some trail marking off to the right heading up a steep heather slope. Do not trust this; It is way off route, and it makes for scary scrambling when wet. The true route is much less steep and follows up into the bowl, nearly to the end where the cliffs give way to scree, dirt and talus field. There are normally good cairns here. This gains the ridge where broad grassy flanks offer specatular views of Mount Dione and the rest of the Tantalus Range.