I have read that OMS and Freemidi can work together.
I can't imagine where you read that.
OMS and FreeMIDI are very similar apps designed to interface the Mac with the external MIDI world. They both take control of the Mac serial port(s) and then connect them to a MIDI interface.
OMS does have a setup to allow "non-OMS" apps to use the ports but it does that by simply releasing control of them when you enable a non-OMS app. That might allow you to run FreeMIDI separately but I don't think there's a comparable setup for FreeMIDI so you would have to actually quit FreeMIDI to get back to OMS. Sounds like a lot of trouble and a likely extension conflict between them.
The question needs to be asked: "Why try to run the two simultaneously to begin with?"
They both do the same thing…almost exactly. The unique capability you get with either is the control software for the MIDI interfaces. The Studio 3,4,5 etc. Opcode units with OMS or the MIDI Express, MIDI Timepiece etc. MOTU units with FreeMIDI.
If you're using Performer and a MOTU interface, it makes sense to use FreeMIDI - everything works well within the MOTU "ecosystem".
Anything else you're very probably better off with OMS.
MacTron can tell you if you need a specific OMS version to work with MIDI Manager.