Well, assuming that your Okidata printer is HP Compatible and has a duplexor unit built in...
Create a printcode on the printcode table that contains ...
$1b &l1S
That is ell one after the "&" in the code
If you want to turn duplex OFF, then $1b &l0S
Put that code in at the end of your init code, probably printcode 3,or as the format init code on the options page in define output.
BTW, the HP 2015 should work with filePro OK. It is rather lacking in in font choice, but a $50 or so font DIMM from HP should fix you up. THe HP 1015 IS brain dead.