A sensitive liquid chromatography-tandem mass spectrometry method was developed and validated for the quantitation of oxymorphone (OM) in human whole blood and liver. Sample preparation was done by solid-phase extraction, using deuterated OM as the internal standard. Separation was achieved using a Waters Aquity UPLC HSS T3 column. Analysis utilized positive electrospray ionization and multiple reaction monitoring. As part of the validation, studies were conducted to determine potential interference, selectivity, ion suppression/enhancement and carryover. Calibration model, limit of detection (LOD), lower limit of quantitation (LLOQ), precision and accuracy were also established. The linear range of the method was 2-500 ng/mL in blood and 5-500 ng/g in the liver. The LOD and LLOQ were 2 ng/mL for blood and 5 ng/g for the liver. Blood and/or liver specimens from 30 cases were analyzed. OM concentrations ranged from 23 to 554 ng/mL ( , n = 26) in blood and 48 to 1740 ng/g ( , n = 30) in the liver.