- O'Beirne's experiment's sphincter
- O'Dwyer's method
- O'Dwyer's method's tube
- o.d.
- O.L.A.
- O.L.P.
- Oak Orchard Acid Springs
- oarialgia
- oarium
- oat-cell sarcoma
- oat-shaped cell
- obcecation
- Obdiplostemonous
- obducent cartilage
- obeliac
- obelion
- Obermayers test
- Obermeler's spirillum
- Obermueller's test
- Oberst's method