Office 365 is unable to assign meeting reminder times different than the default.  The default reminder time in your Outlook Client is also different than the reminder time assigned to your Office 365 account.

To change the default reminder associated with Appointment.one scheduled meetings login to office.outlook.com and:

• Navigate to your calendar
• Click the settings gear and select "Options"
• Navigate to the "Reminders" section and set the default reminder

Alternately, if you reconnect your account as an Exchange type rather than Office 365 this bypasses the Microsoft bug allowing you to select any reminder time you desire.