I just had a similar issue. I have a feature length doc and I couldn't export an OMF. It would stop about 3/4 the way and give me errors. After some reading I found out that OMF only deals with mono files. So, what I did was I went through ever clip in my timeline and changed each stereo clip to mono. Here's the process:
I'm on a Mac too by the way.
In the timeline, match frame any clip that appears to not be mono. That clip will show up in your viewer. In the file menu, click on Clip/Modify/Audio Channels. Near the top where it says Preset, click the drop down menu and click Mono. A warning will come up to tell you that other clips wont be affected. That's ok, click OK. Then replace the clip in the timeline with the one in the viewer. You should get two audio clips in two audio tracks.
the worst thing you can do, I think, is to have a stereo audio clip in one track. I think Premiers circuts blow up when it sees that.
Hope that helps.