Code: Select all
\documentclass{article}
\usepackage{mathtools}
\usepackage{siunitx}
\DeclarePairedDelimiterXPP\BigOSI[2]%
{\mathcal{O}}{(}{)}{}%
{\SI{#1}{#2}}
\begin{document}
\begin{align}
X&=x+\BigOSI{}{\epsilon^{3/2}}\\
Y&=y+\BigOSI{}{{\bar{r}}^{3/2}}
\end{align}
\end{document}