I believe you reserve via the Crook County Park and rec. I made a reservation by calling their office. Prineville reservoir is different.
You can make a reservation at: www.ccprd.org Select camping and then choose which campground you would like. Crook County RV Park Ochoco Lake Campground Haystack Reservoir Campground
https://oregonstateparks.org/index.cfm?do=parkPage.dsp_parkPage&parkId=26
Call the forest service.
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