How can I create a company-wide shared calendar in Office365 or SharePoint that can be accessed in Outlook and OWA?
I am looking for the simplest MS-supported option.
One way of accomplishing half of your question is to make a Calendar in a SharePoint site that everyone in your organization has access to. Assuming someone can view the calendar app in said SharePoint site, the "Connect to Outlook" button in the Calendar tab of the Calendar app will sync the calendar with Outlook 2013. I didn't have any luck getting the calendar to show up in OWA, though, so there may be a better answer out there.
Another option In O365 if you are using OWA you can create calendars directly in the interface and share the calendar.
Go to the Calendar interface and create a new calendar with the "+" icon
In the upper right hand corner - select Share
Add the users or groups you will share with and they will get an email. The email will contain a link to the calendar as well as a link that will add it Outlook Web Access.