LaTeX3 Expansion without \exp_after:wN

LaTeX3 reduces the need to use \expandafter/\exp_after:wN tremendously. But what is the LaTeX3 way for this problem?

\documentclass{article}
\usepackage{expl3}
\begin{document}
  \ExplSyntaxOn
  \tl_new:N \__a
  \tl_new:N \__b
  \tl_set:Nn \__a { \emph{emph} }
  \tl_set:No \__b { \exp_after:wN \overset 
                    \exp_after:wN { 
                    \exp_after:wN o 
                    \exp_after:wN } 
                    \exp_after:wN { \__a }  }
  \tl_show:N \__b % target: b = \overset{o}{\emph{emph}}
\end{document}