What is OFL and how can I remove it?

Official documentation is here: http://www.aes.org/e-lib/browse.cfm?elib=12531

The file shouldn't be zero-length. That likely means something else is going on other than OFL. Our native MP3 plug-in (AmioMp3) supports OFL. I don't think there's an easy way to export an MP3 file by using a different plug-in.

This seems to be default save behaviour as far as I can tell. OFL gets tagged on to all CBR MP3 files. Have you tried saving VBR to see if you still get the OFL tag?

You're right that there is not much about this tag online at all and it can cause issues with radio automation systems and even some podcast hosts in my experience. 

I wish I had a more comprehensive answer on OFL but agree it would be good to have an easy way to switch it on and off. 

I think that this is about Original File Length metadata, and relates to what it says here: .OFL file extension, and I'm not sure how Audition handles this - or even if it does at all. Audition's MP3 handling is essentially bought in from Fraunhofer (well it was - I'm assuming that it still is). If this is just a metadata file, then it should point to the MP3 file that accompanies it, and run it at the correct duration, which it easily might not have done otherwise. You might wish to quiz the originators of these files as to exactly how they're creating them, and whether they have actually sent you all the information necessary to open them correctly.