The voltages arrows are misplaced with respect to the components, in this case a resistor.
\begin{figure}[!h]
\centering
\ctikzset { bipoles/length=1cm}
\begin{circuitikz} [european ,scale=0.65]
\draw
(5,0) to[short]
(0,0) to[V = $V_{supply}$]
(0,3) to[generic=${Shunt}$]
(5,3);
\draw
(5,0) to[R,mirror,v>=$v_{o}$,i<=$i_o$] (5,3);
\draw (6,0) to[R,mirror,v>=$v_{o}$,i<=$i_o$] (8,0);
\end{circuitikz}
\label{fig:OI_model}
\caption{The Output Impedance Model for SCCs. }
\end{figure}
See the result: