Thanks all - so maybe my inclusion of a MIDI track is what's causing the OMF rendering to crash.  I'll try removing that or potentially render that MIDI drum track down to individual WAV files per everyone's suggestion above. 
 
That said, how do you export a MIDI file to share with someone?  I don't recall seeing anything specific to MIDI in the export dialog.
 
It's interesting OMF doesn't support MIDI - sounds like it doesn't buy much over just exporting the individual tracks to WAV files then. 
 
 
 
 
 

1 way is to select the midi clip by clicking on it, then simply drag/drop the midi into any folder..

I have a client that insists on OMF. I had the same crashing problem with no midi tracks, only audio. My fix was to export all tracks as 48 24 then drag them back in. All tracks are consolidated wav files, no pieces. OMF now works.
 
Next time I may just try bouncing all tracks, so there's only 1 long clip per track, may that'll work. So karma did you ever get it to work?

Yes, I found doing things in steps helped a lot:
- save the original version, then remove all FX in the effects bin, enable the master FX bypass (for prochannel effects)
- remove any MIDI tracks from the project
- bounced down all comped tracks so each had one long clip per track.  I think the key was for me to do them one at a time (or a couple tracks at a time) as opposed to letting the OMF do all these during OMF creation, this way I could find the track which was causing an issue.  I did have one or two tracks in a project which seemed to have issues when bouncing down - not sure if there was corruption in one of the WAVs, but once I deleted the comp for that track and bounced it down again, all seemed to work well.
- OMF has a 2 Gbyte filesize limit.  I found the resulting error code wasn't very intuitive - it just said something like 'error creating OMF', but didn't mention anything about file size.  So I would watch the file size increment in file manager as the OMF was being created, so I knew whether or not that was an issue.  If I exceeded the 2 GB limit (which I did in many cases, since my projects were 40-50 tracks of 24 bit / 96 KHz audio), I needed to break the project down into two OMFs by deleting half the tracks, creating an OMF, then reopening the file, deleting the other half and creating another OMF.
 
Hope that helps.
Russ

I use OMF all the time.  I wish I saw this thread earlier I think I could have helped. 
 
Yes, bounce and freeze all your midi and tracks.  Audio Clips with fades, overlaps and edits will cause you trouble. 
 
I have not had any problems with effects or midi being in the project, just realize they won't transfer.
 
Before i render the OMF file I "Remove All Silence".  This will make your OMF smaller.  I don't use the default setting, I set the DB level at 90db to make sure i get all the subtle fade ins and outs
 

Ohh and by the way...many times I thought Sonar crashed at the end of the render...and it did, but the OMF was fine despite the crash.   Check it out.

Jimbo 88
Before i render the OMF file I "Remove All Silence".  This will make your OMF smaller.  I don't use the default setting, I set the DB level at 90db to make sure i get all the subtle fade ins and outs

Agreed, I do this as well - I should have included this in my response above.  Thanks!
 

Jimbo 88
Ohh and by the way...many times I thought Sonar crashed at the end of the render...and it did, but the OMF was fine despite the crash.   Check it out.

Yes, the same thing happened to me once or twice.  It was strange, but I was happy to see the OMF was ok.

Thanks for chiming in Jimbo. It's nice we have someone using OMF a lot. I hadn't used it in probably a decade and now I have a client that insists on it.
 
 It worked on 2 songs without a problem, except having Sonar crash after it made the OMF. Then they wanted the OMF not saved in 1 file, but having the audio saved externally. Then the crash happened before the OMF was created.
 
This is when I went to some extreme measures of starting with the original Platinum default template with no plugins and importing the audio from the other files into this new file. I still had crashes until I exported all the files and imported them back in as continuous wav files. Next time I do this I'll have lots of tips and tricks from everyone here.