If you are trying to add an event and it doesn't show it in your Opus One, please try to verify all steps below:

1) Did you already add the Calendar on your Calendar app first?
I mean here:
2) Make sure you select your Default Calendar on Opus One
You need to go to your Opus One>Preferences>Calendar>Default Calendar

3) Make sure your Calendar is not Hide in your Opus One and Calendar App

4) Verify permission on your Calendar
In order to do that, please go to "system Preferences">Security&Privacy>Calendar
Make sure Opus One is selected.