Opportunistic locking is part of the Windows client file caching mechanism. Samba implements opportunistic locking as a server-side component of the client caching mechanism. Samba/CIFS doesn’t play nice with NFS, so if you’re in a mixed environment where some windows machines access and modify the same files that Linux or Solaris touch through NFS, then disable the oplocks. This is important for things like database files to avoid corruption!

Fedora 10 Default Services