Click on the Menu tab in Opera and select Mail and Chat Accounts...
Select Email and click Next
Enter the name you want to appear when a recipient receives your email. Then type in your email address, in this example firstname.lastname@example.org, example.com being your domain hosted by One.com. Then click Next.
Enter your full email address as the Login name and the password you have created in the Control Panel
Check the box for IMAP.
Then click Next
Enter imap.one.com as Incoming server and check Use secure connection (TLS).
For Outgoing server type send.one.com and check Use secure connection (TLS)
Then click Finish.
You are now almost done. Click on the Menu tab in Opera and select Mail and Chat Accounts...
Select the account you are configuring and click Edit...
Click on the tab Servers
For Incoming IMAP server the port number should be 993. For Outgoing SMTP server the port number should be 465. Authentication should be set to AUTH LOGIN
Click OK and Close to finish.