Background Thoracic trauma is commonplace and accounts for 50–70% of the injuries found in severe trauma. Little information is available in the literature as to timing of endotracheal intubation. The main objective of this study was to assess the accuracy of the ROX index in predicting successful standard oxygen (SO) therapy outcomes, and in pre-empting intubation. Methods Patient selection included all thoracic trauma patients treated with standard oxygen who were admitted to a Level I trauma center between January 1, 2013 and April 30, 2020. Successful standard SO outcomes were defined as non-requirement of invasive mechanical ventilation within the 7 first days after thoracic trauma. Results One hundred seventy one patients were studied, 49 of whom required endotracheal intubation for acute respiratory distress (28.6%). A ROX index score ≤ 12.85 yielded an area under the ROC curve of 0.88 with a 95% CI [0.80–0.94], 81.63sensitivity, 95%CI [0.69–0.91] and 88.52 specificity, 95%CI [0.82–0.94] involving a Youden index of 0.70. Patients with a median ROX index greater than 12.85 within the initial 24 h were less likely to require mechanical ventilation within the initial 7 days of thoracic trauma. Conclusion We have shown that a ROX index greater than 12.85 at 24 h was linked to successful standard oxygen therapy outcomes in critical thoracic trauma patients. It is our belief that an early low ROX index in the initial phase of trauma should heighten vigilance on the part of the attending intensivist, who has a duty to optimize management.