Thanks for your reply.
I see in the Cisco document they have mentioned as below -
"HA support for OUI update: HA link must be up while downloading the OUI file to the Active controller, so that the OUI update gets applied to the Standby controller as well."
But the document hasn't mentioned anything about rebooting both the controllers.
Just noticed this older discussion and have something more to add. TAC opened https://bst.cloudapps.cisco.com/bugsearch/bug/CSCvt48313 for me a little while back - but the conclusion is that this will never be fixed in AireOS. 9800 IOS-XE support for OUI update is planned but not yet supported - currently only updated by loading a new software version. For AireOS the TAC explanation goes like this: "Currently the WLC has the OUIs in an XML file and when we upload a new OUI file, the WLC checks the existing OUIs in the xml file and compares with the oui.txt files and only the ones already in the xml will be updated.
If there are new OUIs that do not exist in the internal XML, they will not be added to the xml file."
If you badly need a new OUI then you'll have to open a TAC case and request it and then Cisco might consider including it with a future release, but their preferred/recommended solution is to use ISE profiling not local profiling. So 'updating' the local OUI file generally has little, if any, effect if you're already on a recent version of code.
No reboot required on recent code versions and OUI update is synced to the HA SSO standby.
ps. also see https://linuxnet.ca/ieee/oui/ which tries to clean up mistakes in the original IEEE file. I recall finding some mistakes it had missed which I reported to them but never got any response and haven't checked it since then.
As at right now (7th August 2020) that would be:
Basically the latest available code will have the latest OUI file.