Introduction. Current definition and staging of acute kidney injury (AKI) considers both serum creatinine (sCr) and urinary output (UO) alterations. However, the relevance of oliguria-based criteria is disputed. Objectives. To determine the contribution of oliguria, as defined by KDIGO criteria, to AKI diagnosis, severity assessment and mortality prediction. Methods. We conducted a cohort study including all adult patients admitted within our multi-disciplinary intensive care unit between January 1st 2010 and June 15th 2020. Daily sCr and hourly UO measurements along with socio-demographic characteristics and severity scores were extracted from our electronic medical charts. Long-term mortality was assessed by cross-referencing our database with the Swiss national death registry. We determined the onset and severity of AKI according to KDIGO classification using UO and sCr criteria separately and assessed their agreement. Using a multivariable model accounting for baseline characteristics, severity scores and sCr stages, we evaluated the relative influence of UO criteria on 90-day mortality. Sensitivity analyses were conducted to assess the impact of missing sCr, body weight and UO values. Results. Among the 15′620 patients included in the study [10′330 (66.1%) males, median age 65.0 years (IQR, 53.0-75.0), median SAPS score 40.0 (IQR, 30.0-53.0), median follow-up 67.0 months (IQR, 34.0-100.0)], 12′143 (77.7%) fulfilled AKI criteria. SCr and UO criteria had poor agreement on AKI diagnosis and staging (Cohen's weighted kappa = 0.36, 95% CI 0.34-0.37, p < 0.001). Compared to the isolated use of sCr criteria, consideration of UO criteria enabled to identify AKI in 5′630 (36.0%) patients. Those patients had a higher 90-day mortality than no-AKI patients (respectively 12.9% and 8.3%, p < 0.001). On multivariable analysis accounting for sCr stage, comorbidities and illness severity, UO stage 2 and 3 were associated with a higher 90-day mortality [OR 2.4 (1.6-3.8), p < 0.001, and 6.2 (3.7-10.5), p < 0.001, respectively]. These results remained significant in all sensitivity analyses. Conclusion. Oliguria lasting more than 12 h (KDIGO stage 2 or 3) has major diagnostic and prognostic implications, irrespective of sCr elevations.