Google Calendar erroneously identifies Opera as mobile

Solution 1:

You could try changing your "User Preferences" to always show the Desktop version of a website.

1. If you type opera:config in the address bar you will be presented with the "Preferences Editor".
2. Expand the "User Prefs" option and copy the below into the "Custom User Agent" field:

For Mac/Linux:

Opera/9.80 (Macintosh; Intel Mac OS X; U; en) Presto/2.2.15 Version/10.10

For Windows:

Opera/9.80 (Windows NT 6.1; U; en) Presto/2.2.15 Version/10.10

Finally, scroll to the bottom, click Save and then OK on the next dialog to confirm the changes.

Source

Solution 2:

Use the Google Calendar's desktop URL instead of the mobile version's URL.