\documentclass{article}
\usepackage[paperheight=80mm,paperwidth=95mm, margin=5mm]{geometry}
\usepackage{feynmp-auto}
\addtolength{\oddsidemargin}{-10mm}
\addtolength{\evensidemargin}{-10mm}
% commas in feynmp sometimes lead to strange errors, use \comma instead
\DeclareMathSymbol{\comma}{\mathpunct}{letters}{"3B}

\begin{document}\LARGE
    \centering
    \begin{fmffile}{O1_a_diagram} \centering 
        \begin{fmfgraph*}(300,180)
            \fmfstraight   
            \fmfleft{l0,l1,l2,l3,l4}            
            \fmfright{r0,r1,r2,r3,r4}
            
            \fmf{phantom}{l1,w1,r1}
            \fmf{phantom}{l3,v1,r3}
            \fmffreeze
            
            \fmf{fermion,label=$u$,label.side=left}{l4,v1}
            \fmf{fermion,label=$s$,label.side=left}{v1,r4}
            
            \fmf{photon,label=$W$,tension=1.0,label.side=right}{v1,w1}               
            \fmf{fermion,label=$d$,label.side=right}{l0,w1}           
            \fmf{fermion,label=$u$,label.side=right}{w1,r0}
            
            
        \end{fmfgraph*}
    \end{fmffile}
    \pagestyle{empty}
\end{document}