To shed light on the efficacy of oxygen therapy in pulmonary arterial hypertension (PAH) when hypoxemia is not present, we report seven years of observational data for a female patient recorded between February 2012 and February 2019 when she was aged 7.8–14.8 years. The patient was born with an atrial septal defect (closed spontaneously at 2.3 years) and ventricular septal defect (surgically repaired at 8.3 years) and then diagnosed with PAH at 8.9 years . The patient was prescribed bosentan soon after diagnosis and for the next 4.8 years, during which a first phase of oxygen therapy (nocturnal) was trialed for 2.8 years. Mean pulmonary arterial pressure (mPAP) and systolic PAP (sPAP) remained stable and at mild levels when oxygen was administered, but then increased progressively to severe levels over two years without oxygen. This coincided with worsening right ventricular pathology during the later part of this period without oxygen. Re-initiation of more intensive oxygen therapy while the patient was still on bosentan and before pharmocotherapy was changed coincided with a large and rapid fall in sPAP, confirmed by right heart catheterization measurements of mPAP. During this entire observation period, the patient remained normoxemic. These observations challenge the notion that oxygen therapy should be restricted to patients with hypoxemia and strengthen calls for further study of oxygen therapy in PAH.