documentclass{beamer}

usepackage[default, scale=0.95]{opensans}
usepackage{siunitx}

setbeamerfont{math text}{series=fontseries{l}}    % does not change anything
setbeamerfont{normal text}{series=fontseries{l}}
AtBeginDocument{usebeamerfont{normal text}}

begin{document}
    begin{frame}{Some Math and Text}
        here is some text in OpenSans light \
        but $math,mode$ is not in light: si{microsecond}
    end{frame}
end{document}