Typically, garbage points to a mis-match in the settings between the spooler/port and the attached printer. Confirm the "ditty-rp -a ttyx##" results match the settings applied on the Okidata printer with regards to baud rate, parity, flow control, etc...
Are you able to cat data directly to the printer and have it print properly?
cat /etc/inittab > /dev/ttyx##
Also, confirm the port on the PortServer is configured for use with RealPort. I'm not sure what PortServer model you have, however "prn" works on all models. Telnet to the PortServer and login as root:
#> set port dev=prn range=port#
#> kill tty=(port#)
If there is still a problem, you should contact Digi Tech. Support to pursue this further: