- olympic national park
- o'keeffe
- or anything
- or belonging to
- omerta
- otherworldly
- on one's head
- open order
- ovarian ligament
- ovarian steroid biosynthesis
- optic disk
- open collector
- ocular palsy
- oxyopia
- one who exploits
- one who constantly hangs upon
- occipital pole
- orthogonal analysis
- orthogonal list
- ostrava
- opheodrys vernalis
- osteochondrosarcoma
- old gril
- one who stands first
- out of fun
- of words
- on points
- optical track
- optical scanning
- orpheus
- orange juice
- open bubble
- ophiuchus
- opheodrys aestivus
- ophelic acid
- of unknown name
- olfactory brain
- osteolytic
- one's aspirations to be shattered
- oliver cromwell
- olivelike
- oil suspension
- orthocranic
- one who presents (a gift)
- onomasticon
- one who takes to an evil course
- omnidirectional
- one who wields a foil or cudgel with skill
- organ neurosis
- osmoreceptor