To follow-up my posting, I now find that using WGRIB works
for all my downloaded OSO files! I just realized that the files for which WGRIB was giving errors were those files which had been ftp'd in a batch process while those for which it was working successfully were those I had ftp'd manually - so I added a "binary" command to the batch ftp and WGRIB is no longer giving error messages. So the problem appears to have been in the downloaded files themselves, not in the use of WGRIB. Mea culpa.