\PassOptionsToPackage{round,authoryear}{natbib}
\documentclass[
twoside,openright,titlepage,numbers=noenddot,headinclude,
cleardoublepage=empty,
BCOR=5mm,paper=b5,fontsize=9.5pt,
DIV=14,
dutch,american,
% Languages
]{scrbook}
\usepackage{natbib}
\usepackage{lipsum}
\begin{document}
\chapter{MWE}
\label{MWE}
%\noindent
\lipsum \cite{companion}
\bibliographystyle{aa}
\bibliography{biblatex-examples}
\end{document}