O-Benzyl-N-tert-butoxycarbonyl ω-Hydroxy Norfentanyl-d5 is a deuterated synthetic intermediate and labeled analog used primarily as an internal standard in analytical chemistry, particularly liquid chromatography-mass spectrometry (LC-MS/MS), for the precise quantification of ω-hydroxy norfentanyl, a key metabolite of fentanyl, in biological matrices such as plasma, urine, and tissues. This compound supports pharmacokinetic, pharmacodynamic, and toxicological studies of fentanyl and its derivatives, facilitating research into opioid metabolism, drug monitoring in therapeutic and forensic contexts, detection of abuse, and assessment of overdose risks. The deuterium labeling (≥98 atom % D) ensures stability and minimizes isotopic interference, enhancing accuracy in metabolic profiling and development of analytical methods for safer opioid use.