Exclusive lock to a share

We have H700 and have created a SMB share on it

This SMB share is being accessed by 2 different systems lets call it SYSA and SYSB.

SYSA writes files to it, while SYSB just reads from this share. SYSA writes mxf files, which are growing files.

Now we need to establish an exclusive lock mechanism while SYSA is writing, so until the file is complete and the file handle is open SYSB shouldn't be able to read it. 

At the moment whats happening is SYSA is writing, while SYSB can immediately read and copy data across resulting in partial data on SYSB side.

I have checked we do have OPLOCKS enabled for this share, but the exclusive lock still doesnt work. What more do we need to do to realize this lock