The drive mapping does not matter as long as the permissions are at least read-only.
What matters is that the SYBASE environment variable points to the directory containing the OCS-12_5 folder and the 'ini' folder containing the 'sql.ini' file. In addition, you must make sure that the SYBASE\OCS-12_5\bin and dll folders are on your machine's path. On Windows, the sql.ini is always found under the %SYBASE%\ini folder.
You should also check your SQLCA settings. The DBMS setting should be "SYC" in order to use OpenClient.
Perhaps others have a different SYBASE environment setup than yourself. If the OpenClient dlls and sql.ini are not part of the deployment, that might account for the issues.
Also, as you have hidden the server name in dsedit and the sql.ini, are you using the IP address or DNS name? Perhaps your DNS name resolution or \Windows\system32\drivers\etc\hosts has a host entry pointing the wrong address that other users do not have.
Chris