Since you are using beamer
, you have \insertshortauthor
and \insertauthor
:
\documentclass{beamer}
\author[O. Castillo]{\'Oscar Castillo}
\begin{document}
\begin{frame}
\insertshortauthor
\insertauthor
\end{frame}
\end{document}

After a comment, it seems that the idea is to use this to generate some meta-data information for the PDF; in this case one could use \beamer@shortauthor
:
\documentclass{beamer}
\title{The Title}
\author[O. Castillo]{\'Oscar Castillo}
\makeatletter
\hypersetup{pdfauthor={\beamer@shortauthor}}
\makeatother
\begin{document}
\begin{frame}
test
\end{frame}
\end{document}