Poor "O" never allowed to be a variable.
Poor "O"! It's not his fault that his cousin 0 and he look so much alike!
There are games that won't use "I" or "l" for similar reasons...
"O(O)": Approximately the origin.
Next paper I write, I'm letting o=1 and l=0.
Yarr, stick it to the man and use confusing notation.
Also, don't use 'd' because it's reserved for differentials...
would you call O.o a not-face?
I wonder what comes after the greek letters
Algebraic Number Theory owes a great deal to Germans and uses certain German letters: their D for example.