Oreo is Androids latest version release, and as with every release, there are always going to be kinks and issues that need to be ironed out in the following maintenance update.
A considerable amount of users have been reporting that the Google Clock App is crashing. As a result of this, the moment a pre-set alarm goes off, the App Crashes and goes silent, causing many of these unsuspecting users to run late to work. You can find these over at Reddit.
Google Clock App
If you are running Android Oreo, you might want to check to make sure that your Alarm app is working as intended. An easy way to do this is say OK, Google, set alarm for 1 minute, and then wait to hear it go off.
If your Google App is indeed crashing, you should install another alarm clock app until a fix is figured out or Google acknowledges that its an issue.
Source | Via