At present (931209), O doesn't know about disulfides. The following solution is due to Laurent Maveyraud:

If it is for taking pictures, you have the possibility to draw a stick with using ODL commands. Create a file disulfide.odl which contains:

 begin_object disu   ! or whatever you want
   colour green      ! if your sulfurs are green
   stick molname resnum1 SG molname resnum2 SG radius
 end_object

and draw this from O with:

 O > draw_object disulfide.odl

A more complicated solution is to make a new residue of just the two sulphurs and make a connectivity for them.