Latin Capital Letter O
Perhaps the most visually elemental of all Latin capitals, uppercase 'O' is a closed curve — one of the few letters whose glyph shares its shape with the digit zero, creating an ambiguity that has shaped font design and coding conventions for decades. It is the 15th letter of the Latin alphabet, sitting at ASCII code point 79 (0x4F), exactly 32 below its lowercase counterpart 'o' at 111 (0x6F), maintaining the single-bit case offset (bit 5) shared by all ASCII A–Z / a–z pairs. As one of five English vowels, its uppercase form appears frequently at sentence boundaries and in acronyms.
Technical Details
Code Example
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// Big O complexity examples// O(1) - constant: array[0]// O(n) - linear: for loop// O(n²) - quadratic: nested loops// O(log n) - logarithmic: binary search
Common Usage
Science and mathematics: Uppercase 'O' is the element symbol for oxygen (atomic number 8), one of the most referenced symbols in chemistry and biochemistry. In computer science, Big O notation — O(n), O(log n), O(n²) — uses an uppercase letter to describe asymptotic upper bounds on algorithmic complexity, though technically the symbol derives from the German 'Ordnung' (order) rather than the Latin letter. In medicine, blood type O denotes the universal donor group in the ABO system, making 'O' one of the most consequential single letters in clinical practice.
Zero–O disambiguation: The visual similarity between uppercase 'O' (0x4F) and the digit '0' (0x30) has driven lasting design decisions across computing. Slashed zero (0̸), dotted zero, and the distinctive oval-vs-round contrast in fonts like Consolas and Menlo all exist to separate the two glyphs. In legacy systems such as IATA flight codes and NATO callsigns, strict conventions govern which symbol is permitted in each position to prevent misreads.
Octal notation marker: In Python 3, Rust, and several other modern languages, the prefix 0o (zero followed by lowercase 'o') marks an octal literal — e.g., 0o755 for Unix file permissions. The choice of 'o' for octal was deliberate to avoid confusion with the older C-style leading-zero convention (0755), though it reintroduces the very zero–O ambiguity it sought to reduce when read in certain fonts.
Diacritical variants: Uppercase 'O' supports one of the richest diacritical families in the Latin script — Ò, Ó, Ô, Ö, Õ, Ø, Ō, Ő, Ǫ among others. In Swedish, Finnish, and Turkish, Ö is sorted as a separate letter at the end of the alphabet rather than as a variant of O. Similarly, Ø is a distinct letter in Danish and Norwegian. Hungarian treats Ő (double acute) as a separate alphabetic entry, making 'O' one of the most subdivision-heavy base letters across European orthographies.
Science and mathematics: Uppercase 'O' is the element symbol for oxygen (atomic number 8), one of the most referenced symbols in chemistry and biochemistry. In computer science, Big O notation — O(n), O(log n), O(n²) — uses an uppercase letter to describe asymptotic upper bounds on algorithmic complexity, though technically the symbol derives from the German 'Ordnung' (order) rather than the Latin letter. In medicine, blood type O denotes the universal donor group in the ABO system, making 'O' one of the most consequential single letters in clinical practice.
Zero–O disambiguation: The visual similarity between uppercase 'O' (0x4F) and the digit '0' (0x30) has driven lasting design decisions across computing. Slashed zero (0̸), dotted zero, and the distinctive oval-vs-round contrast in fonts like Consolas and Menlo all exist to separate the two glyphs. In legacy systems such as IATA flight codes and NATO callsigns, strict conventions govern which symbol is permitted in each position to prevent misreads.
Octal notation marker: In Python 3, Rust, and several other modern languages, the prefix 0o (zero followed by lowercase 'o') marks an octal literal — e.g., 0o755 for Unix file permissions. The choice of 'o' for octal was deliberate to avoid confusion with the older C-style leading-zero convention (0755), though it reintroduces the very zero–O ambiguity it sought to reduce when read in certain fonts.
Diacritical variants: Uppercase 'O' supports one of the richest diacritical families in the Latin script — Ò, Ó, Ô, Ö, Õ, Ø, Ō, Ő, Ǫ among others. In Swedish, Finnish, and Turkish, Ö is sorted as a separate letter at the end of the alphabet rather than as a variant of O. Similarly, Ø is a distinct letter in Danish and Norwegian. Hungarian treats Ő (double acute) as a separate alphabetic entry, making 'O' one of the most subdivision-heavy base letters across European orthographies.