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Background: The effect of supplemental oxygen therapy in patients with intermediate-risk pulmonary embolism (PE) who do not have hypoxemia at baseline is uncertain., Research Question: Does supplemental oxygen improve echocardiographic parameters in nonhypoxemic patients with intermediate-risk PE?, Study Design and Methods: This pilot trial randomly assigned nonhypoxemic patients with stable PE and echocardiographic right ventricle (RV) enlargement to receive anticoagulation plus supplemental oxygen for the first 48 h vs anticoagulation alone. The primary outcome was normal echocardiographic RV size 48 h after randomization. Secondary efficacy outcomes were the numerical change in the RV to left ventricle (LV) diameter ratio measured 48 h and 7 days after randomization with respect to the baseline ratio measured at inclusion., Results: The study was stopped prematurely because of the COVID-19 pandemic after recruiting 70 patients (mean ± SD age, 67.3 ± 16.1 years; 36 female [51.4%]) with primary outcome data. Forty-eight h after randomization, normalization of the RV size occurred in 14 of the 33 patients (42.4%) assigned to oxygen and in eight of the 37 patients (21.6%) assigned to ambient air (P = .08). In the oxygen group, the mean RV to LV ratio was reduced from 1.28 ± 0.28 at baseline to 1.01 ± 0.16 at 48 h (P < .001); in the ambient air group, mean RV to LV ratios were 1.21 ± 0.18 at baseline and 1.08 ± 0.19 at 48 h (P < .01). At 90 days, one major bleeding event and one death (both in the ambient air group) had occurred., Interpretation: In analyses limited by a small number of enrollees, compared with ambient air, supplemental oxygen did not significantly increase the proportion of patients with nonhypoxemic intermediate-risk PE whose RV to LV ratio normalized after 48 h of treatment. This pilot trial showed improvement in some ancillary efficacy outcomes and provides support for a definitive clinical outcomes trial., Trial Registry: ClinicalTrials.gov; No.: NCT04003116; URL: www., Clinicaltrials: gov., Competing Interests: Financial/Nonfinancial Disclosures The authors have reported to CHEST the following: L. J.-P. reports personal fees from Bayer Hispania, Actelion, Rovi, Pfizer, Menarini, and Leo Pharma, outside the submitted work. R. O.-C. has served as an advisor or consultant for Leo Pharma, Actelion, Bayer Hispania, and Sanofi; as a speaker or a member of a speakers’ bureau for Leo Pharma, Sanofi, and Rovi; and has received grants for clinical research from Leo Pharma and Bayer Hispania. P. R.-A. has served as an advisor or consultant for Leo Pharma, Pfizer, and Viatris; has served as a speaker or a member of a speakers’ bureau for Bristol-Myers Squibb, Daiichi Sankyo, Leo Pharma, ROVI, and Viatris; and has received grants for clinical research from ROVI. M. M. has served as an advisor or consultant for Leo Pharma and Sanofi; has served as a speaker or a member of a speakers’ bureau for Bristol-Myers Squibb and Sanofi; and has received grants for clinical research from Sanofi and ROVI. B. B. reports that he is a consulting expert, on behalf of the plaintiff, for litigation related to two specific brand models of inferior vena cava filters. D. J. has served as an advisor or consultant for Bayer HealthCare Pharmaceuticals, Bristol-Myers Squibb, Daiichi Sankyo, Leo Pharma, Pfizer, ROVI, and Sanofi; has served as a speaker or a member of a speakers’ bureau for Bayer HealthCare Pharmaceuticals, Bristol-Myers Squibb, Daiichi Sankyo, Leo Pharma, ROVI, and Sanofi; and has received grants for clinical research from Daiichi Sankyo, Sanofi, and ROVI. None declared (D. B., D. D., C. R., J. M., A. R., J. L. L., R. L., L. C., A. M.)., (Copyright © 2023 American College of Chest Physicians. Published by Elsevier Inc. All rights reserved.)