Oxycodone, an opioid commonly used to treat pain in humans, has the potential to be abused in racehorses to enhance their performance. To understand the pharmacokinetics of oxycodone and its metabolites in horses, as well as to detect the illegal use of oxycodone in racehorses, a method for quantification and confirmation of oxycodone and its metabolites is needed. In this study, we developed and validated an ultra-high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) method that can simultaneously quantify and confirm oxycodone and eight metabolites in equine urine. Samples were subjected to enzymatic hydrolysis and then liquid-liquid extraction using ethyl acetate. The analyte separation was achieved on a Hypersil Gold C18 sub-2 µm column and analytes were detected on a triple quadrupole mass spectrometer. The limit of detection (LOD) and lower limit of quantification (LLOQ) were 25-50 pg/mL and 100 pg/mL, respectively. Excellent linearity of the calibration curves was observed over a range of 100-10000 pg/mL for all nine analytes. Retention time, signal-to-noise ratio, and product ion ratios were utilized as confirmation criteria, with the limits of confirmation (LOC) ranging from 100 to 250 pg/mL. The data from a pilot pharmacokinetic (PK) study suggested that oxycodone metabolites have longer detection periods in equine urine compared to oxycodone itself; thus, the detection of metabolites in equine urine extends the ability to detect oxycodone exposure in racehorses.