Introduction: Polysomnography is the gold standard for diagnosing paediatric obstructive sleep apnoea (OSA); however, limited availability restricts access in many low-income settings. Level 4 portable oximetry may serve as a practical interim diagnostic alternative. This study reports findings from an expanded cohort, building on previously presented preliminary data. Objectives: To evaluate the diagnostic utility of level 4 portable oximetry for paediatric OSA and to assess associations between clinical assessment, Sleep-Related Breathing Disorder Paediatric Sleep Questionnaire (SRBD-PSQ) scores, and oximetry parameters. Methods: A cohort study was conducted in Galle, Sri Lanka in 2025, including 50 children with suspected sleep-disordered breathing. Participants underwent clinical evaluation, SRBD-PSQ assessment, early morning blood gas analysis, and overnight continuous pulse oximetry . The cohort included equal males and females (1:1), aged 4 months–15 years (mean 8 years); 51% had underlying syndromes. Results: 50.0% of the participants screened positive on the SRBD-PSQ. Adequate-quality oximetry data were obtained in 76% of studies (mean duration 6 hours). OSA was identified in 57 .9% of participants, predominantly mild disease (36.8%). No significant correlations were identified. Management guided by oximetry findings included weight reduction (22.7%), noninvasive ventilation (9.1%), and surgical referral (6.8%), and monitoring with interval studies (45.5%). Conclusions: Level 4 portable oximetry demonstrated meaningful diagnostic utility for paediatric OSA in this expanded cohort and represents a feasible interim tool to improve access to timely care in resource-limited Asia-Pacific settings.