Just found this here :
The location of the client registry entry for opportunistic locking
has changed from the location in Microsoft Windows NT. In later
versions of Windows, you can disable opportunistic locking by setting
the following registry entry to 1:
OplocksDisabled REG_DWORD 0 or 1
Default: 0 (not disabled) Note The OplocksDisabled entry configures Windows clients to
request or not to request opportunistic locks on a remote file.
You can also deny the granting of opportunistic locks by setting the
following registry entry to 0:
EnableOplocks REG_DWORD 0 or 1
Default: 1 (enabled) Note The EnableOplocks entry configures Windows-based servers to
allow or to deny opportunistic locks on local files. These servers include
workstations that share files.