How to get a small letter version of \mathcal{O} [duplicate]

Question

I am trying to produce a small letter version of \mathcal{O} which looks the same, just smaller - like "O" and "o". However \mathcal{o} produces a very different symbol. Has anyone an idea how to manage this?

Answer 1

% arara: pdflatex

\documentclass{article}
\usepackage{graphicx}
\newcommand\smallO{
  \mathchoice
    {{\scriptstyle\mathcal{O}}}% \displaystyle
    {{\scriptstyle\mathcal{O}}}% \textstyle
    {{\scriptscriptstyle\mathcal{O}}}% \scriptstyle
    {\scalebox{.7}{$\scriptscriptstyle\mathcal{O}$}}%\scriptscriptstyle
  }

\begin{document}
$\mathcal{O} \smallO$
\end{document}

Answer 2

\mathcal{} uses only CAPITAL LETTER ALPHABETS. To use small letter alphabets, you can use \mathscr{}, which also supports CAPITAL LETTER ALPHABETS. To use \mathscr{}, you need to include the package unicode-mathand use the font XITS Math and compile the document using xelatex or lualatex.

Here is an MWE:

\documentclass{article}
\usepackage{unicode-math}
\setmathfont{XITS Math}
\begin{document}
$\mathcal{O}$ with \verb+\mathcal{}+ \par
$\mathscr{o}$ with \verb+\mathscr{}+ \par
$\mathscr{O}$ with \verb+\mathscr{}+
\end{document}