English Words Starting With O

• onus

  nounC1
  /ˈəʊ.nəs/A responsibility, burden, or duty that one is obliged to undertake or prove.
• onus probandi

  nounC2
  /ˌəʊnəs prəʊˈbændiː/The burden or duty of proving a disputed assertion, claim, or fact.
• onychia

  nounVery low
  /əʊˈnɪkɪə/Inflammation or infection of the nail bed.
• onychophagia

  nounVery Rare / Specialist
  /ˌɒn.ɪ.kəʊˈfeɪ.dʒi.ə/The medical or psychological term for the compulsive habit of biting one's fingernails.
• onychophoran

  nounVery Low
  /ˌɒnɪˈkɒfərən/A small, worm-like invertebrate animal with soft, velvety skin, stubby legs, and claws.
• onyx

  adjectivenounC1
  /ˈɒnɪks/A semiprecious stone, a variety of chalcedony, typically with alternating parallel layers of black and white.
• onyx marble

  nounLow
  /ˈɒn.ɪks ˈmɑː.bəl/A type of marble characterized by bands of different colors, especially layers of green, white, or brown, often used for decorative purposes.
• onyxis

  nounRare
  /ɒˈnɪksɪs/A condition of the fingernails or toenails, specifically characterized by the nail growing into the flesh at the side, causing inflammation and pain.
• oo gauge

  nounC1
  /ˌəʊˈəʊ ɡeɪdʒ/A specific scale for model railways, primarily used in the United Kingdom, where a scale of 1:76 is used to model 4ft 8.5in standard gauge track, representing 16.5 mm track width.
• ooblast

  nounVery Rare / Specialized
  /ˈəʊə(ʊ)blɑːst/A specialized embryonic cell that develops into an egg cell.
• oocyst

  nounC2 - Very low frequency; specialised technical term.
  /ˈəʊəsɪst/A thick-walled structure produced by some protozoan parasites, particularly within the Apicomplexa phylum, containing the zygote and serving as a stage in the life cycle for transmission and survival in the environment.
• oocyte

  nounC2
  /ˈəʊ.ə.saɪt/An immature egg cell; the cell in an ovary that undergoes meiosis to form an ovum.
• oodles

  nounplural-nounB2
  /ˈuː.dəlz/a large amount or number of something; lots.
• oof

  interjectionnounLow
  /ʊf/An exclamation expressing physical discomfort, sharp pain, or exhaustion.
• oogamete

  nounVery Low (Specialised)
  /ˈəʊəɡəmiːt/A female gamete or egg cell, especially in plants, algae, or fungi, that is large, non-motile, and fertilized by a smaller, motile male gamete (spermatozoid).
• oogamy

  nounVery Low (C2+)
  /əʊˈɒɡəmi/A form of sexual reproduction involving the fusion of a large, non-motile egg (ovum) and a small, motile sperm.
• oogenesis

  nounC1
  /ˌəʊə(ʊ)ˈdʒɛnɪsɪs/The biological process of egg cell formation in female organisms.
• oogonium

  nounVery Low
  /ˌəʊəˈɡəʊnɪəm/The female reproductive structure or cell in certain algae, fungi, and oomycetes that produces and contains one or more eggs.
• ooh

  interjectionnounverb-intransitiveMedium to High (in spoken/informal contexts)
  /uː/An exclamation of surprise, delight, pleasure, admiration, or (sometimes) pain.
• ookinesis

  nounVery Low (C2+ Technical)
  /ˌəʊ.ə(ʊ).kɪˈniː.sɪs/The process or phenomenon of egg activation or cell division in an egg (ovum) following fertilization or parthenogenetic stimulus.

Showing 1261–1280 of 3187 words.