1. Stiff shrub having oblong entire leaves and dense cymes of yellow flowers
   - Hypericum prolificum, Hypericum spathulatum

Type of: St John's wort

• Onychophora
• onychophoran
• onychosis
• onymous
• onyx
• onyx marble
• onyxis
• oocyte
• oodles
• oogenesis
• ooh
• oolite
• oology
• oolong
• Oom Paul Kruger
• oomph
• Oomycetes
• oooh
• oophorectomy
• oophoritis
• oophorosalpingectomy
• oops
• oops-a-daisy
• Oort
• Oort cloud