Migrated users cant see on-prem resource mailboxes via OWA

Any resources they use will have to be migrated to o365 as well.
Any shared mailboxes that they accessed will have to be migrated to o365 as well.
They need to be on the same premise

what version of exchange are you using and are you in a full hybrid and is your external autodiscover pointing at on-prem exchange and does your firewall allow the recommended microsoft IP addresses to communicate with your hybrid server. you should be able to still schedule and on-prem room even after the mailbox is migrated. you dont have to migrate rooms to o365 to continue to use them. you should be able to use them in outlook and OWA. I just tested this successfully.

Also just a note, shared mailboxes can also be access cross-premise just like the resource mailboxes. it depends on your hybrid setup and if everything is setup properly. we use all these features in our hybrid environment. make sure that the OU for those resources are also being synced via AD Connect so that O365 will be aware they exist in your on-prem environment.

I've had a ticket open with MSFT for over two weeks, it just seems as though I have to set up room lists in order for these to show up.
Users can add the resources if they search and add them from the GAL as Attendees, just not as rooms or resources.

Since MSFT hasn't been able to get back to me, I'm going to try to put the resources into room lists based on their locations or floors etc since there is a 100 object limit.