- Obesity
- Obesity – children
- Obsessive compulsive disorder
- Obstructive sleep apnoea
- Occupational overuse syndrome (OOS)
- Omega-3 and fish oil supplements
- Onychomycosis (fungal nail infection)
- Oral health – baby
- Oral health – children
- Oral health – children – 1 to 2 years
- Oral health – children topics
- Oral health – children: 2 to 3 years
- Oral health – taking care of your teeth
- Oral health – tooth decay
- Oral thrush
- Osgood-Schlatter disease
- Osteoarthritis
- Osteoporosis
- Otitis externa
- Otitis media
- Outbreaks and epidemics
- Ovarian cancer
- Ovarian cysts
- Overactive thyroid