A. With Exclusive Oplock, if a file is opened in a non-exclusive (deny
none) mode, the redirector requests an opportunistic lock of the entire file. As
long as no other process has the file open, the server will grant this oplock,
giving the redirector exclusive access to the specified file. This will allow
the redirector to perform read-ahead, write-behind, and lock caching, as long as
no other process tries to open the file.
When a second process attempts to open the file, the original owner will be asked to Break Oplock or Break to Level II Oplock. At that point, the redirector must invalidate cached data, flush writes and locks, and release the oplock, or close the file.
Opportunistic Locking level II, provides a method for granting read access to a file by more than one workstation, and these workstations can cache read data locally (read-ahead). As long as no station writes to the file, multiple stations can have the file open with level II oplock.
For more information see http://support.microsoft.com/support/kb/articles/Q129/2/02.asp has LOADS of information about tuning both the server and workstation.