Unfortunately, most NASes take the all-on or all-off approach to oplocks, for various reasons. One vendor that I checked with pointed to problems with certain versions of Samba and also certain applications, so has defaulted its product to opplocks-off, but offers an add-on for users who want them enabled. This same vendor plans to change the default to enabling oplocks in an upcoming firmware update, but will provide an option in its management GUI to disable them.
Another NAS vendor defaults its products to oplocks-on for all shares, but offers a "database optimization" mode to disable oplocks on all shares. Yet another vendor started with having its NAS default to enabling oplocks, but changed the default to disable after experiencing problems oplocks getting stuck on when clients disconnected before closing an opened file. This vendor is currently evaluating alternative approaches.
Since it is Windows clients (or to be specific, clients running the SMB protocol) that set the oplocks, one would hope that this feature could be disabled there. This Microsoft KB article describes the Registry edits to do just that, except for Vista. (If you need the regedits for Win NT, use this KB article.) To further complicate the issue, Microsoft introduced a new version of the SMB protocolSMB2in Vista, which does not allow oplocks to be disabled.
So, what's the bottom line? Don Capps' advice above seems to provide a good set set of recommendations. Unfortunately, most consumer NASes don't provide share-by-share control of oplocks. So the next best thing seems to be the path that many NAS vendors are taking; disabling oplocks. This seems to minimize application-specific NAS problems and has the benefit of improved throughput.
Since vendors don't provide information on their oplock approach, however, you may have to experiment for yourself by using the Windows Registry hacks referenced above and doing your own testing. I'm interested in what you find, so post a comment and share your results.