There's an experimental way to do it. Goto the URL opera://flags/#sidebar-site-panel, enable the flag, restart Opera, click the 3 dots at the bottom of the sidebar, scroll down to "Custom Site Panels" and add the URL for the page you want.
Custom site panels are strict though and do not allow redirects across domains. If the page has a link to a different domain, clicking it will open the link in a tab instead of in a panel. And, if you use a service like Outlook.com where it redirects you across domains to log you in, you'll have to log in with a regular tab first to set the session cookie and then click the reload button for the panel. Then the panel will load fine as long as the session is valid.
Also, you can't set the icon for the panel in the sidebar. And, you can only have a few custom site panels.
But, it's something.