Can you keep a kokanee in Oregon?

No minimum size, ten a day, open all year unless a specific water says otherwise.

ODFW source page

What we left out on purpose

• The lower reaches of the Deschutes River (near its mouth at the Columbia River) are designated a thermal angling sanctuary with possible additional rules from July 15 – September 15, and separate salmon/steelhead seasons apply below Sherars Falls — none of this applies to the Lake Billy Chinook-to-Benham-Falls stretch this site tracks, so it is not modelled here.
• Sturgeon, salmon and steelhead run their own genuinely complex statewide and Columbia River Basin rules (annual tags, Combined Angling Tags, catch-and-release-only sturgeon) that don't apply to this stretch of the Deschutes and are not modelled.
• Oregon's other six zones (Northwest, Southwest, Willamette, Southeast, Snake River, Columbia River, Marine) and their own statewide defaults are not modelled — this site tracks only the Central Zone stretch of the Deschutes River.
• Bull trout are closed to angling everywhere in the Central Zone unless a specific water says otherwise; not modelled since it's a closed, protected species rather than a real bag figure.