documentclass{article}
usepackage{stringstrings}
newcommand{opind}[2]{
  convertchar[q]{OPS#2}{_}{}
  #1 & #2 & pageref{thestring} \
}
begin{document}
begin{table}[h]
  begin{tabular}{l l l}
    opind{1}{FO_O} \
  end{tabular}
end{table}
clearpage
label{OPSFOO} FOO \
end{document}

begin{table}[h]
  begin{tabular}{l l l}
     convertchar[q]{OPSFO_O}{_}{}
     1 & FO_O & pageref{thestring} \
  end{tabular}
end{table}

convertchar[q]{OPSFO_O}{_}{}
begin{table}[h]
 begin{tabular}{l l l}
   1 & FO_O & pageref{thestring} \
 end{tabular}
end{table}