OO doesn’t seem to hono mfenced
\documentclass{article}
\begin{document}
\[
\sin\left(x\right)\]
\end{document}
\Configure{left}
{}
{\HCode{<\a:mathml mrow>\expandafter
\ifx \csname \string#1:lr del\endcsname\relax
<\a:mathml mo form="prefix" >#1</\a:mathml mo\space>\else
\expandafter\ifx \csname \string#1:lr del\endcsname\empty \else
<\a:mathml mo form="prefix" >\csname
\string#1:lr del\endcsname</\a:mathml mo>\fi
\fi<\a:mathml mrow>}}
-_-_-
\Configure{right}
{\HCode{</\a:mathml mrow>\expandafter
\ifx \csname \string#1:lr del\endcsname\relax
<\a:mathml mo form="postfix" >#1</\a:mathml mo>\else
\expandafter\ifx \csname \string#1:lr del\endcsname\empty \else
<\a:mathml mo form="postfix" >\csname
\string#1:lr del\endcsname</\a:mathml mo>\fi
\fi
}}
{\HCode{</\a:mathml mrow>}}
-_-_-