The Origins Space Telescope’s orders-of-magnitude improvement over the scientific capabilities of prior infrared (IR) missions is based on its cold telescope (4.5 K) combined with low-noise far-IR detectors and ultra-stable mid-IR detectors. A number of trades were made in favor of a design with staged cooling, avoiding warm objects in the cold zones and minimizing the number of deployments and their complexity. The result is an architecture that is easily to analyze thermally and straightforward to test in existing facilities.