Objectives/background: Sleep laboratory polysomnography (PSG) is the gold standard for obstructive sleep apnea (OSA) diagnosis in infants, but its access remains limited. Another simple and widely used tool, oximetry-capnography can provide information on the presence of desaturations and alveolar hypoventilation. However, its reliability is debated. This study aimed at examining its use in determining OSA severity in infants. Patients/methods: This retrospective study was conducted in a sleep unit in a tertiary hospital, in infants < 4 months old with clinical signs of OSA or Pierre Robin Sequence (PRS) who underwent a one-night PSG coupled with oximetry-capnography. Results: Among the 78 infants included (median [IQR] age: 61 [45-89] days at PSG), 44 presented with PRS, and 34 presented with isolated airway obstruction. The clinical, sleep and respiratory characteristics were not significantly different between the two subgroups. In the entire cohort, 63.5% had severe OSA. Median OAHI was 14.5/h [7.4-5.9], Spo2 was 97.4% [96.5-98.1], and PtCO2 was 41.1 mmHg [38.3-44.9]. The optimal threshold to predict OAHI > 10/h was 6/h for OD3% (sensitivity 95.7%, specificity 51.9%) and 2/h for OD4% (sensitivity 95.7%, specificity 48.1%). Conclusion: Whereas transcutaneous capnography does not appear to be sufficient in predicting severe OSA in infants < 4 months old with PRS or clinical signs of OSA, oximetry may be a useful alternative for the screening of severe OSA in infants in the absence of PSG.