- Obenix
- Obesity
- Obsessive-compulsive disorder (OCD)
- Office of Applied Studies
- Office of National Drug Control Policy
- Office on Smoking and Health
- Online Gamers Anonymous (OLGA)
- Online therapy
- Opiate derivative
- Opiate painkillers
- Opiates
- Opioids
- Opium
- Opium tincture
- Oramorph
- Other specified feeding or eating disorder (OSFED)
- Over-the-counter (OTC) medication
- Overdose
- Overeaters Anonymous
- Oxycodone
- OxyContin