# How to write the code of this character?

I am writing my B.Sc. thesis. But, I have no idea how to write the following character:

I was trying by $\mathcal{O}$, but this is not the same I want.

This is an addendum to Sebastiano's answer, who correctly identified the font. However, I personally always load it with the mathrsfs package since that way \mathcal retains its original definition, and you can get the "strange" O with \mathscr{O}. In particular you also get a nice Lagrange L via \mathscr{L} without losing the ability to produce a luminosity (say) L with \mathcal{L}.

\documentclass{article}
\usepackage{mathrsfs}
\begin{document}
Using \verb|mathrsfs| to produce
$\mathscr{O}_X$
allows us to retain the original definition of \verb|\mathcal|,
$\mathscr{O} \quad\mbox{vs.}\quad \mathcal{O}\qquad\mbox{and}\qquad \mathscr{L} \quad\mbox{vs.}\quad \mathcal{L}\;.$
\end{document}


• Thank you very much for to have put my name into your answer 😊. Into my book I have different calligraphics font, \mathscr{L} and \mathcal{L}. My best regards and best wishes. – Sebastiano Dec 24 '19 at 12:29

The request symbol is into calrsfs package. Here there is a screenshot of the symbol.

\documentclass[a4paper,12pt]{article}
\usepackage{calrsfs}
\begin{document}
$\mathcal{O}_X$
\end{document}


As an addendum, I would recommend loading extra math alphabets through mathalpha, which provides a consistent interface and scaling—if you are using legacy 8-bit fonts. If you can use the modern toolchain with unicode-math, even better. This gives you a consistent interface to scale the alphabets and load them as either \mathscr or \mathcal.

The documentation for mathalpha also comes with specimens of dozens of math alphabets for you to compare. You might, for example, prefer a less heavily slanted version, such as the one from rsfso.