You won't get one - if what you mean is one that will produce a listing of OEM Product Keys against machines (manufactured before 2011/2, at least).
1) Before Windows 8, OEM Licenses were simply stickers on the machines - the installed Key (and hence PID) was a generic one, and was easily counterfeited and the system hacked to accept it. You therefore would need to conduct a physical audit of each
machine to find the COA Sticker and Product Key relating to that machine, and then relate that to the machine ID, and the in-use license (which would presumably be either a KMS or MAK license). Hopefully your purchasing system would have been such that
this record was made at time of delivery, or of installing your own image.
2) Win 8 or above have the OEM Key embedded in the BIOS - you would have to query the BIOS on each machine, and pull the Key from that. I've never seen such a network app - but it's quite possible that such does exist, as it's relatively easy to do.
