On my USB to serial converter, the are a couple of LEDs that tell you if messages are being sent on the out and in lines. Does yours have the same? If so, do you see those LEDs flash at all when you send messages?
I would also suggest that you try to get one direction working at a time. Fix up your sending loop so that you toggle a led on and off with the note on/off that you send. Then verify that you get that message in MIDIOX by monitoring the input window. If you don't work on that first as you are more in control of timing and message.
Then also check the config for MIDIOX. You need to make sure that it is actually using the USB interface for output. The LEDs on the cable would be a good indicator, but you can also check the config to make sure your device paths are set up correctly.
I think OMNI does not work with the midi library. Check the comments on the header file.
An obvious one, but make sure the jumper on the olimex board is replaced after you upload, or you will get nowhere.