Interior of a set notation

The latex kernel contains the \mathring accent for that:

Edit :

You might prefer to use the \ring command from mathabx. Here is a code to use use it without replacing all maths fonts: it defines the \abxring mathaccent.

\DeclareFontFamily{U}{mathb}{\hyphenchar\font45}
\DeclareFontShape{U}{mathb}{m}{n}{ <-6> matha5 <6-7> matha6 <7-8>
mathb7 <8-9> mathb8 <9-10> mathb9 <10-12> mathb10 <12-> mathb12 }{}
\DeclareSymbolFont{mathb}{U}{mathb}{m}{n}

\DeclareMathAccent{\abxring}{0}{mathb}{"38}

\DeclareFontFamily{U}{mathb}{\hyphenchar\font45}
\DeclareFontShape{U}{mathb}{m}{n}{ <-6> matha5 <6-7> matha6 <7-8>
mathb7 <8-9> mathb8 <9-10> mathb9 <10-12> mathb10 <12-> mathb12 }{}
\DeclareSymbolFont{mathb}{U}{mathb}{m}{n}

The symbol seems to be an upright “o”; in order to raise it more than it would be with $A^{\mathrm{o}}$, you can define a macro.

\documentclass{article}

\newcommand{\interior}[1]{%
  {\kern0pt#1}^{\mathrm{o}}%
}

\begin{document}

$\interior{A} \interior{B}$

\end{document}

I'd use the first, but take your pick:-)

\documentclass{article}

\begin{document}


\[
A^\mathrm{o}
\quad
A\strut^\mathrm{o}
\quad
A\mkern-1mu\vrule width0pt height 1em^\mathrm{o}
\quad
A\mkern-1mu{\vrule width0pt height 2ex}^\mathrm{o}
\]

\end{document}