It would be great to have the ability to have teams be notified via slack if a user's on-call schedule starts or ends. This would be similar to how users can be notified via push when their schedule starts/ends. 

Additionally, sending slack notifications when an override is placed.

Workaround:
Where available in the Integrations tab of the Channel or DM, Slack allows you to create an email address for a channel or DM.  

In the User's Notification tab in Opsgenie, this Slack email can then be added as an additional email Contact Method which can then be used in any of the User's notification rules including Schedule Start.

Currently, the Schedule start message does not identify the user name in the notification email, but if it does, then sending the Schedule start message to a shared Slack channel would make sense.