| Octreotide | Endocrinology | LC/MS/MS | 50-4000 pg/mL | Human EDTA Plasma |
| Octreotide | Endocrinology | LC/MS/MS | 0.25-5 ng/mL | Rat EDTA Plasma |
| O-desmethyltramadol | Pain | LC/MS/MS | 1-100 ng/mL | Human Plasma |
| Ofloxacin | Infectious Disease | HPLC/FLR | 5-5000 ng/mL | Human EDTA Plasma |
| o-hydroxy-atorvastatin | Cardiovascular | LC/MS/MS | 0.25-100 ng/mL ; 0.25-20 ng/mL ; 0.25-100 ng/mL | Human Sodium Heparin Plasma |
| Olanzapine | Central Nervous System Agents | LC/MS/MS | 0.1-20 ng/mL | Human EDTA Plasma |
| Olmesartan | Cardiovascular | LC/MS/MS | 3-1500 ng/mL | Human Plasma |
| Omeprazole | Gastrointestinal | LC/MS/MS | 5-1000 ng/mL | Human EDTA Plasma |
| Omeprazole | Gastrointestinal | LC/MS/MS | 2-1000 ng/ml | Human ETDA K2 Plasma |
| Ondansetron | Gastrointestinal | LC/MS/MS | 0.5-200 ng/mL | Human EDTA Plasma |
| Ondansetron | Gastrointestinal | LC/MS | 10-1000 ng/mL | Human Lithium Heparin Plasma |
| Opipramol | Central Nervous System Agents | LC/MS/MS | 100-40000 pg/mL | Human EDTA Plasma |
| Orlistat | Endocrinology/Metabolism | LC/MS/MS | 10-4000 pg/mL | Human EDTA Plasma |
| Oxandrolone | Endocrinology | LC/MS/MS | 0.2-20 ng/mL | Human EDTA Plasma |
| Oxcarbazepine; 10-Hydroxycarbamazepine | Central Nervous System Agents | LC/MS/MS | 4-2000 ng/mL; 20-10000ng/mL | Human EDTA Plasma |
| Oxcarbazepine; 10-Hydroxycarbamazepine | Central Nervous System Agents | LC/MS/MS | 5-2500 ng/mL ; 30-15000 ng/mL | Human EDTA Plasma |
| Oxybutazone | Pain | HPLC/UV | 0.1-50 µg/mL | Horse Plasma |
| Oxybutynin; N-Desethyloxybutynin | Genitourinary | LC/MS/MS | 50-10000 pg/mL ; 100-20000 pg/mL | Human Sodium Heparinized Plasma |
| Oxybutynin; N-Desethyloxybutynin | Genitourinary | LC/MS/MS | 50-10000 pg/mL | Human EDTA Plasma |
| Oxycodone | Pain | LC/MS/MS | 0.25-125 ng/mL | Human EDTA Plasma |
| Oxycodone | Pain | LC/MS/MS | 0.25-90 ng/mL | Human EDTA Plasma |
| Oxycodone | Pain | LC/MS/MS | 100-25000 ng/mL | Human Plasma |
| Oxycodone metabolite: Nor-oxymorphone | Pain | LC/MS/MS | 0.25-50 ng/mL | Human Plasma |
| Oxymorphone | Pain | LC/MS/MS | 0.1-20 ng/mL | Human EDTA Plasma |
| Oxymorphone metabolite: 6-OH Oxymorphone | Pain | LC/MS/MS | 0.1-20 ng/mL | Human EDTA Plasma |
| Oxypurinol | Rheumatology | LC/MS/MS | 25-2000 ng/mL | Human EDTA Plasma |