Omicron

The big-O symbol was introduced by Paul Bachmann in 1894 and popularized by Edmund Landau in 1909, originally standing for "order of" ("Ordnung") and being thus a Latin letter, was apparently viewed by Donald Knuth in 1976[3] as a capital Omicron, probably in reference to his definition of the symbol (capital) Omega.

There were several systems for writing numbers in Greek; the most common form used in late classical era used omicron (either upper or lower case) to represent the value 70.

[a][b] Omicron is used to designate the fifteenth star in a constellation group, its ordinal placement an irregular function of both magnitude and position.

Instead, the name of the letter O in classical Attic times was simply the long version of its characteric sound: οὖ (pronounced /o:/) (that of Ω was likewise ὦ).

[9][d] By the second and third centuries CE, distinctions between long and short vowels began to disappear in pronunciation, leading to confusion between O and Ω in spelling.

[9] During the early outbreak of the Omicron variant of COVID-19, many people unfamiliar with the entire Greek alphabet (or simply lacking the ability to pronounce or sound out words using phonetics) mispronounced Omicron as "Omnicron" due to the unfamiliarity of the letter, and the use of the prefix "Omni-" in many words.