FYI, copy/paste from Incident# SP66473 in Office 365's System Health Dashboard :
Current Status: We've identified an issue in which users are observing OneDrive for Business sync client issues on Mac OS X. We've determined that an unexpected problem caused impact to occur during a regularly scheduled systems update. Engineers are deploying a fix on the affected systems to remediate impact.
User Impact: Users may observe that the OneDrive for Business sync client for Mac OS X stops responding when a background task attempts to synchronize files. Users can force quit the application and remove it from the list of login items to avoid interruptions due to client crashes while the fix is being deployed. While we're focused on remediation, users can access their files by using the OneDrive for Business website.
Scope of Impact: A few customers have reported this issue, and our analysis indicates that this issue is specific to Mac users who have the OneDrive for Business sync client installed.
Start Time: Sunday, June 26, 2016, at 4:02 AM UTC
Preliminary Root Cause: An unexpected problem caused impact to occur during a regularly scheduled systems update.
Next Update by: Wednesday, June 1, 2016, at 11:00 PM UTC