O

/ou/
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Thân thiện
Definition

  1. Noun:

    • The letter O: The 15th letter of the modern English alphabet and the Roman alphabet.
    • The number zero: Used in speech and writing, especially to avoid confusion with the letter 'O'.
    • A blood type: The blood group whose red blood cells carry neither the A nor B antigens.
    • The chemical element Oxygen: A nonmetallic, bivalent element, a colorless, odorless, tasteless, diatomic gas that constitutes about 21% of the Earth's atmosphere.

  2. Interjection:

    • An expression of emotion: Used to express surprise, pain, desire, or to address someone directly. (Often written as 'oh', but 'o' is a poetic or archaic variant).
Usage Examples

  • Noun:

    • The word "book" starts with the letter 'o' in some languages, but not in English.
    • Please enter your password: it's capital O as in "Oscar", not the number zero.
    • People with blood type O are sometimes called universal donors.
    • O is essential for respiration and combustion.

  • Interjection:

    • "O, say can you see..." (from the U.S. national anthem).
    • O, I wish I had known sooner!
Advanced Usage

  • "O" as a direct address (vocative case): Used in formal, poetic, or religious language to directly invoke a person or thing.

    • "Hear me, O king!"
    • "O Captain! my Captain!" (from the poem by Walt Whitman).

  • In mathematics and science: Represents order of magnitude (Big O notation) or the origin point on a graph (point O).

Variants and Related Words
  • Oh: The more common modern spelling of the interjection.
  • Oxygen (O₂): The molecular form of the element.
  • Type O: The common term for the O blood group.
Synonyms
  • Zero, nought, nil (for the number).
  • Oxygen (for the element).
  • Oh, alas, ah (for the interjection, depending on context).
Related Phrases and Idioms
  • O'clock: Of the clock; used to specify the hour.
    • The meeting is at three o'clock.
  • O ring: A gasket or seal in the shape of a ring.
    • The mechanic replaced the faulty O ring.
  • O positive/O negative: Subtypes of the O blood group based on the Rhesus factor.
Noun
  1. the blood group whose red cells carry neither the A nor B antigens
    • people with type O blood are universal donors
  2. the 15th letter of the Roman alphabet
  3. a nonmetallic bivalent element that is normally a colorless odorless tasteless nonflammable diatomic gas; constitutes 21 percent of the atmosphere by volume; the most abundant element in the earth's crust

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