I came across an issue today wherein a user whose mailbox was hosted on Office 365 attempted to use OWA to book a meeting On Premise, and was told that there were no rooms available. The list simply didn’t contain any rooms. The Room Mailboxes were synchronised to Office 365 using the AADSync tool, however OWA knew nothing about them. When using Outlook, the Rooms were shown correctly, so this was just an issue with OWA, not with Room Mailboxes per se.
After a little digging, I found the following KB article: https://support.microsoft.com/en-us/kb/2904381 which explains that a Room List should be created and synchronised to Office 365 to get this working in OWA.
However the PowerShell cmdlet in the KB article fell a little short as I had over 100 Rooms to add into the Distribution Group. This was the PowerShell I ended up using:
$members=Get-Mailbox -RecipientTypeDetails RoomMailbox
New-DistributionGroup -Name "RoomList" -RoomList -Members $Members
I then moved the newly created Distribution Group into the correct OU and performed a Directory Sync. The RoomList showed up instantly, but it took 5-10 minutes for it to become populated. Once this was all done, the Rooms were available!