使用\begin{itemize}[label=$\circ$]
:
\documentclass{article}
\usepackage{enumitem}
\begin{document}
\begin{itemize}
\item a
\begin{itemize}[label=$\circ$]
\item a1
\item a2 % circle skipped here
\item a3
\end{itemize}
\item b
\end{itemize}
\end{document}