Dave Goldman blogged how to change the OAB Generation Temp generation folder to a different location.
In Exchange 2007 SP2 we added a registry key called OALGEN_REG_V4_ALT_TEMP_FILES_LOCATION which will allow you to change the temporary storage location for the OAB Version 4 temporary files. This registry key will *ONLY* work for OAB Version 4. OAB versions 2 and 3’s temporary files will still go to the %temp% directory. If this registry key is not present on the OAB’s generating system we will revert back to the %temp% directory.
Being that OAB Version 4 is generated slightly different and uses different compression algorithms we can allow for a larger number of users far exceeded what we could generate on legacy systems.