Aggregate availability from multiple calendars

Only the "primary " calendar is used indeed (you can have multiple calendar folders in the same mailbox but few/busy will only reflect the default one). So the only solution is to populate the appointments into the O365 calendar as well. As to how to do it, there are multiple methods. Adding the O365 account as an attendee is an easy solution and hardly takes more than few seconds, and you should even automate it via rules or macros. Similarly, you can get the messages directly into your O365 mailbox by configuring some form of forwarding, or using the Connected accounts functionality. Copy/pasting calendar items is also easy enough to do if you switch to a list view.