\documentclass{oblivoir}

\usepackage{chngcntr}

\usepackage{bm}

\counterwithin*{equation}{section}

\counterwithin*{equation}{subsection}

\begin{document}

\section{A}

\begin{equation} \int_0^1 ax dx \end{equation}

\begin{equation} \int_0^1 ax dx \end{equation}

\begin{equation} \int_0^1 ax dx \end{equation}

\section{B}

\begin{equation} \int_0^1 ax dx \end{equation}

\begin{equation} \int_0^1 ax dx \end{equation}

\begin{equation} \int_0^1 ax dx \end{equation}

\section{C}

\begin{equation} \int_0^1 ax dx \end{equation}

\begin{equation} \int_0^1 ax dx \end{equation}

\begin{equation} \int_0^1 ax dx \end{equation}

질문전에 미리 검색좀 합시다!

$\phi = \mathbf{B} \cdot \mathbf{S}$

$\phi = \bm{B} \cdot \bm{S}$

\end{document}