The Greek letter omicron has been in the news recently, with the World Health Organization giving that letter to the latest variant of COVID-19 (skipping over nu and xi).
I had known for a very long time that Greek had two letter Os (omicron O o and omega Ω ω , corresponding approximately to the sounds in hop and hope), but it took me a long time to learn or figure out that they are literally little O (o + micron) and big O (o + mega) respectively. (Compare Korean ㅓ and ㅗ, the same idea and approximately the same sounds. (I don’t know if Koreans conceptualise ㅓ and ㅗ as being ‘closer’ than, say ㅏ and ㅜ.))
In other contexts, Little O is something mathematical, which I won’t attempt to explain, and Big O means something different to mathematicians, watchers of Japanese anime, writers and readers of erotica (no link, obvs) and fans of Roy Orbison. (Is there any overlap between those categories? Have two people ever had a seriously embarrassing conversation by assuming that the other meant something different?)
(See also the many uses of omicron and omega in the pages linked above.)
PS 10 Dec: Numberphile has a video about some mathematical usages of omicron, which I won’t pretend to understand. I noticed that he pronounced omicron with a short ‘o’ all the time, and omega with a long ‘o’ most of the time, but once or twice with a short ‘o’. I suspect that once Hindu/Arabic numerals came into use in Europe, omicron was less used because it could be mistake for zero. Notice that at 5.37 of the video, the paper they discuss is titled Big omicron and big omega and big theta. Big omicron is literally big little o, and big omega is big big o.