Hi Chrislsb,
Based on the current situation, I would like to confirm whether you selected your Office 365 calendar only when you tried to sync the local calendar to OWA.
If not, please open the calendar on your mobile, uncheck all the calendars and then just check the Office 365 calendar.
After confirming that the calendar only showed is your Office 365 calendar, please sync again and check if the issue persists.
Thanks,
Kid Yang