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A multicenter, retrospective cohort study analyzed 2,645 adult cardiac arrest patients from 208 US ICUs. PaO₂ was measured on multiple occasions within 24 hours of admission. The primary outcome was 28-day all-cause mortality. Generalized additive models and threshold analysis identified inflection points. Conventional cut-off values for PaO₂ were applied for sensitivity analysis. PaO₂ was categorized as hyperoxemia (mild: 100–150 mmHg, moderate: 150–225 mmHg, severe: 225–300 mmHg, extreme: >300 mmHg), normoxemia (60–100 mmHg), or hypoxemia (<60 mmHg). Cox regression and subgroup analyses assessed mortality risk.