CDC Yellow Book
- O'nyong-nyong virus
- obstructive sleep apnea (OSA)
- Oceania [map]
- Ocean Island (Banaba)
- ofatumumab (Arzerra) [table]
- off-label use, of medications
- ofloxacin [table]
- Oman
- ondansetron
- One Health approach to zoonotic illnesses
- Opdivo (nivolumab) [table]
- open itineraries, long-term travelers with
- opportunistic infections
- oral rehydration salts (ORS)
- Orencia (abatacept) [table]
- Organization for Safety, Asepsis, and Prevention
- organ transplant recipients
- Oropouche virus
- Oroya fever
- ORS (oral rehydration salts)
- OSA (obstructive sleep apnea)
- Overseas Security Advisory Council
- over-the-counter medications
- oxygen, supplemental
- oxygen toxicity
- oxyuriasis