In FOG 0.33, the host no longer has an OS. The OS is tied to the image.
I have modified the script in the init.gz to truncate to 15 characters and warn if truncation was necessary. I still have not modified the webUI to limit the field in html, or to truncate the value in the processing of the form data.
I agree this would be nice, and for the most part, I never need over 15 characters. Maybe this could be configurable through the FOG settings for a max host name length in case there was some need to have it longer than 15. There is a limit due to the field size in the datastore also, but that can be queried to make sure the UI or registration scripts are not exceeding that.