As per the OASYS install instructions:

Within the FreeMIDI Configuration Window:

1) Under the "File" Menu, select "FreeMIDI Preferences"
2) Under the MIDI Configuration section you should see (provided the OASYS' drivers were installed correctly) OASYS Ports A and B. Click on the box next to each Port to enable it.
3) Click 'OK'. Now, withing the FreeMIDI Configuration Window you should see the OASYS Ports displayed. They do not need to be connected to anything in this window.

Within the OASYS PCI Editor Window.

1) Under the "Edit" Menu, select "Preference".
2) Click on the MIDI/Global Tab.
3) From the MIDI System pull-down menu select "FreeMIDI".
4) Check the box next to "Editor Responds to OMS/FreeMIDI Controllers" if you desire to use OASYS in a 'stand-alone 'mode. If you are going to use OASYS within a sequencer such as Digital Performer, leave this unselected.


Now OASYS should be setup to run under the FreeMIDI system.