You can add a subscription to your Office 365 Calendar in Full Slate, so those events show in Full Slate...and you can add a subscription to your Full Slate calendar into your Office 365 Calendar.
To add your Office 365 Calendar to Full Slate, follow the instructions listed at http://help.outlook.com/en-us/140/ms.exch.ecp.calendarpublishing.aspx and then paste the resulting URL in Full Slate under Schedule > More > Sync, Import & Export > 'Synchronize Full Slate with an external calendar' > 'Add connection' > 'Subscribe to another external calendar'. Be sure to choose the Full Slate schedule on which you want the subscription.
To add your Full Slate calendar to Office 365, go to the Schedule tab in your Full Slate account. Click More > Sync, Import, Export > 'View your Full Slate calendar in another application', and then copy the "webcal" link. Next, go to your Office 365 Calendar and follow the instructions listed here: http://community.office365.com/en-us/forums/158/t/165283.aspx.
Please be aware that anytime two disparate programs are connected, it's not possible to guarantee 100% accuracy. As a result, we recommend not fully relying on a sync or subscription to operate your business. The safest approach is to check your schedule directly on Full Slate, whether on the web or mobile.