O'Leary Mountain Horde is a 25-Swarmer Horde in Cascade, one of the Horde Killer targets. It opens after We've All Done Things. Clear it for 2,000 XP and 1,600 Trust at Copeland's Camp.
At this size a Horde is a good first try. Find a spot with one way in, such as a ladder, ledge or doorway. Molotovs, a couple of pipe bombs and a shotgun or SMG will do the rest.
Hordes bed down somewhere dark during the day and move out at night to feed and drink. Scout the area in daylight while it sleeps, then pick when to start the fight. If you only need to pass through, give its route a wide berth after dark.
You can ride off, restock and come back. The kill count holds for a while, but stray Swarmers top the Horde back up if you wait too long. Keep your bike fuelled and parked on your exit route, not in the Horde's path.
This Horde chased Deacon up the mountain trails from the day he and Boozer moved into the lookout tower. Killing it is a small payback he wants to share with Boozer.
Cascade Highway Horde
The Cascade Highway Horde is a 50-Swarmer Horde Killer target along the Santiam Highway near the logging camp. It opens after We've All Done Things and clears one of Deacon and Boozer's old danger spots.
Horse Lake Horde
The Horse Lake Horde is a 25-Swarmer Horde Killer target near the Caldera Hydro Dam reservoir in the Cascade. It is one of the smallest Hordes in the game and a good first try.
Little Bear Lake Horde
The Little Bear Lake Horde is a 30-Swarmer Horde Killer target in the Cascade that often blocks the old Belknap Tunnel route. It opens after We've All Done Things and is an easy one to start with.
Logging Camp Infestation
The Logging Camp Infestation is the first nest zone you burn, at the logging camp at the foot of O'Leary Mountain in Cascade. You clear it in Clear Out Those Nests to keep Freakers away from the safehouse while Boozer heals.