Proceed as follows:
1. Open the OMS MIDI Setup dialog in OMS Setup and make sure that
“Run MIDI In Background” is enabled.
2. Launch Reason.
It is important that you launch Reason after making changes to OMS, or
the changes you have made will not be available.
3. Set up the other program so that it transmits MIDI to an OMS IAC
port.
4. In Reason, open the MIDI section of the Preferences dialog.
5. Open the MIDI input pop-up for the MIDI Input port(s) that should receive
the incoming MIDI, and select the IAC port that you set up in
step 3.
Note that the Sequencer Port only receives MIDI on one selected channel
at a time.