Oliguria is a decreased urine output that can be a sign of acute renal failure. The document discusses evaluating and managing oliguria in a postoperative patient. Common causes of postoperative oliguria include fluid loss, decreased renal perfusion, or intrinsic kidney injury. Assessment involves checking for dehydration, obstruction, or acute kidney injury risk factors. Initial management focuses on fluid resuscitation and stopping nephrotoxic drugs. Further treatment depends on whether the cause is pre-renal, renal, or post-renal.