Background: Previous studies have compared CT findings of COVID-19 pneumonia with those of other infections; however, to our knowledge, no studies have included non-infectious organizing pneumonia (OP) as a comparison group. Objective: To compare chest CT features of COVID-19, influenza, and OP using a multireader design, and to assess radiologists' performance in distinguishing between these conditions. Methods: This retrospective study included 150 chest CT examinations in 150 patients (mean age 58±16 years) with diagnosis of COVID-19, influenza, or non-infectious OP (50 randomly selected abnormal CT examinations per diagnosis). Six thoracic radiologists independently assessed CT examinations for 14 individual CT findings and Radiologic Society of North America (RSNA) COVID-19 category and recorded a favored diagnosis. CT characteristics of the three diagnoses were compared using random effects models; readers' diagnostic performance was assessed. Results: COVID-19 pneumonia was significantly different (p<.05) from influenza pneumonia for seven of 14 chest CT findings, though different (p<.05) from OP for 4 of 14 findings [central or diffuse distribution in 10% and 7% of COVID-19 vs 20% and 21% of OP; unilateral distribution in 1% of COVID-19 vs 7% of OP; non-tree-in-bud nodules in 32% of COVID-19 vs 53% of OP; tree-in-bud nodules in 6% of COVID-19 vs 14% of OP]. A total of 70% of cases of COVID-19, 33% of influenza, and 47% of OP had an RSNA COVID-19 category of typical (p<.001). Mean percentage of correct favored diagnoses compared to actual diagnoses was 44% for COVID-19, 29% for influenza, and 39% for OP. Mean diagnostic accuracy of favored diagnoses was 70% for COVID-19 pneumonia and 68% for both influenza and OP. Conclusion: CT findings of COVID-19 substantially overlap with influenza and, to a greater extent, with OP. Radiologists' diagnostic accuracy was low in a study sample containing equal proportions of these three types of pneumonia. Clinical Impact: Recognized challenges in diagnosing COVID-19 by CT are furthered by our observed strong overlap between CT appearances of COVID-19 and OP. This challenge may be particularly evident in clinical settings with substantial proportions of patients with potential causes of OP such as ongoing cancer therapy or autoimmune conditions.