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Platypnoea–orthodeoxia syndrome (POS) is a rare cause of posture-dependent hypoxaemia, usually linked to intracardiac right-to-left shunts. We report an elderly woman with progressive dyspnoea and recurrent falls whose oxygen saturation fell from 95% supine to 84% upright. Imaging showed kyphoscoliosis with partial right hemi-diaphragmatic dysfunction. Supine and upright contrast transthoracic echocardiography demonstrated delayed bubble appearance, excluding an intracardiac shunt and suggesting an intrapulmonary mechanism. CT pulmonary angiography and Tc-99m macroaggregated albumin scintigraphy ruled out pulmonary arteriovenous malformations, indicating posture-dependent ventilation–perfusion mismatch as the cause. Comprehensive geriatric assessment identified frailty and limited physiological reserve, guiding conservative management with pulmonary rehabilitation and fall-prevention measures. This case emphasises the importance of postural pulse oximetry, sequential imaging and geriatric assessment in diagnosing POS in older adults and describes a rare extracardiac mechanism without cardiac or vascular shunting.