documentclass[twoside]{scrartcl}[2015/10/03]
usepackage{showframe}

% my edit:
renewcommandsectionlinesformat[4]{%
ifthispageodd{%
    raggedleftmakebox[0pt][r]{#4}#3}{%
    raggedrightmakebox[0pt][r]{#3}{#4}}
}

usepackage[headwidth=textwithmarginpar]{scrlayer-scrpage}
clearpairofpagestyles
ohead{pagemark}
usepackage{blindtext}
begin{document}
Blinddocument
end{document}