As already mentioned, there are two packages to solve this problem:
- xfrac - typeset fractions in the form
n/d
generally
- faktor - especially to typeset factor structures
Here is a comparison between the \sfrac{n}{d}
and \faktor{n}{d}
commands which also demonstrates how they behave in comparison to normal text:

\documentclass[12pt,preview, border={2pt,2pt,2pt,2pt}]{standalone}
\usepackage[english]{babel}
\usepackage{xfrac}
\usepackage{amsmath}
\usepackage[amsmath,thmmarks,standard]{ntheorem}
\usepackage{faktor}
\begin{document}
\begin{alignat*}{4}
\text{This } &\sfrac{\mathcal{O}_{(V',0)}}{\mathcal{O}_{(V,0)}} &\text{ and this } &\sfrac{\mathcal{S}^n}{\equiv_m} &\text{ and this } &\sfrac{A}{B} &&\text{ is \texttt{sfrac}.}\\
\text{This } &\faktor{\mathcal{O}_{(V',0)}}{\mathcal{O}_{(V,0)}} &\text{ and this } &\faktor{\mathcal{S}^n}{\equiv_m} &\text{ and this } &\faktor{A}{B} &&\text{ is \texttt{faktor}.}
\end{alignat*}
\end{document}