\documentclass{article}
\newcommand{\mysection0}{\section{#1}}
\newcommand{\mysection1}[1]{\section{#1}}
\newcommand{\mysection2}[1][1]{\section{#1}}
\newcommand\mysection3[1][1]{\section{#1}}
\renewcommand{\foo}{\section{#1}}
\providecommand{\bar}{\section{#1}}
\def\baz{\section{#1}}
\DeclareMathOperator{\op}{foo}
\begin{document}
\mysection0{ABC}
\mysection1{EFG}
\mysection2{HIJ}
\end{document}
