\mathrm and \Omega in XeLaTeX

Question

When I write $\mathrm{\Omega}$ and compile it with LaTeX, I get capital omega symbol normally. But when I do the same with XeLaTeX, I get empty symbol. I don't use any special packages or fonts.

Could someone explain me why is this? And how could I achieve the same behavior in XeLaTeX as in LaTeX?

Example of LaTeX file:

\documentclass[10pt]{article}
\usepackage{lmodern}
\usepackage[utf8]{inputenc}

\title{Article}
\author{Name}
\date{}
\begin{document}
   $\text{nothing:} \Omega +  \text{rm} \mathrm{\Omega} + \text{normal} \mathnormal{\Omega}$
\end{document}

Example of XeLaTeX file:

\documentclass[10pt]{article}
\usepackage{xunicode}
\usepackage{fontspec}

\title{Article}
\author{Name}
\date{}
\begin{document}
   $\text{nothing:} \Omega +  \text{rm} \mathrm{\Omega} + \text{normal} \mathnormal{\Omega}$
\end{document}

Answer 1

Use \Omega for upright Ω, and use \varOmega with amsmath package for italic Ω.

\documentclass{article}
\usepackage{amsmath}
\usepackage{fontspec}
\begin{document}

$\Omega \varOmega$

\end{document}

---

Some more explanations:

With fontspec, \mathrm uses text roman fonts by default. However, there is no Ω in Latin Modern Roman font, thus the symbol is missing.

Sometimes, you may want to use no-math option of fontspec to prevent changing the math fonts, if you use some math font packages. However, you should just use correct way to typeset \Omega and \varOmega instead.