Mit dem Tipp `\raisebox{5ex}{$\oplus$}` von @Henri ergibt sich:
\documentclass{article}
\usepackage{chemfig}
\renewcommand*\printatom[1]{\ensuremath{\mathsf{#1}}}
\setchemfig{atom sep = 2.3em,
cram width = 4pt,
lewis sep = 3pt
}
\begin{document}
\begin{center}
\chemname{\chemleft[
\chemfig{R-[:30]\chemabove{\lewis{2,O}}
{\hspace{3.15cm}\oplus}(>:[:-20]R{''})<[:-70]R^{'}}
\chemright]}{\textbf{I}}
\hspace{1.5cm}
\chemname{\chemleft[
\chemfig{H-[:30]\lewis{2,O}(>:[:-20]H)<[:-70]H}
\chemright]\raisebox{5ex}{$\oplus$}}{\textbf{II}}
\end{center}
\end{document}
