The simplest way is to edit the PostScript file that is produced by oplot. In this file, you'll find lines such as:
/Times-Roman findfont 5.38 scalefont setfont
Remove ALL of these EXCEPT THE FIRST. In the first, put the point size you want to use for your labels, e.g.:
/Times-Roman findfont 12 scalefont setfont
for 12 point text labels.