Turns out that \mathcal O matches the picture OP submits; But the concept of big O seems to be represented by \Omicron. At least in wikipeida, big O's friends are Greek. In this (specific) case, knowing that there are thousands of O-like symbols does not help.
– Symbol 1Apr 01 '15 at 03:21
@Symbol1 - If you look up \Omicron in TeX's list of math greek symbols, you'll come up empty -- as it would merely duplicate the Latin character O. (The Romans took over some, but not all, Greek uppercase letters in classical times. Latin-script lowercase letters are a much later invention.) I suppose that one advantage of using \mathcal{O} to denote "of order of" is that it is rather unlikely to ever be confused with 0 (zero).
– MicoApr 01 '15 at 03:45
@Mico Comparing to \mathcal{O}, I would suggest using \mathrm{O} to represent the Greek O since other Greek letters are typeset in upright.
– Symbol 1Apr 01 '15 at 05:00
\mathcal Omatches the picture OP submits; But the concept of big O seems to be represented by\Omicron. At least in wikipeida, big O's friends are Greek. In this (specific) case, knowing that there are thousands of O-like symbols does not help. – Symbol 1 Apr 01 '15 at 03:21\Omicronin TeX's list of math greek symbols, you'll come up empty -- as it would merely duplicate the Latin characterO. (The Romans took over some, but not all, Greek uppercase letters in classical times. Latin-script lowercase letters are a much later invention.) I suppose that one advantage of using\mathcal{O}to denote "of order of" is that it is rather unlikely to ever be confused with0(zero). – Mico Apr 01 '15 at 03:45\mathcal{O}, I would suggest using\mathrm{O}to represent the Greek O since other Greek letters are typeset in upright. – Symbol 1 Apr 01 '15 at 05:00