Hi @Lemarac ,
Please check if the room lists can be seen in OWA, the room list (distribution group) and the list members (rooms) generally would take up to 24 hours to appears in the Outlook Client.
According to my research, this issue may occur if the user doesn't select a room list. A room list must be selected before available rooms are displayed in the Room Finder.
You mentioned that room can be seen in Office 2016. For further research, please provide the specific version of Outlook 365. (File > Office Account > About Outlook) Please make sure to upgrade your client to the latest version.
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