It still should be safe to use it because Opera did say that they will continue to provide security patches for v.12 they just won't add any new features to it. Of coarse it will be their last priority to patch older versions.
The Presto browsing engine has never really been exploited by malware since it isn't as popular as the other browsers like IE, Firefox and Google Chrome.
Just make sure to disable Java plugin and keep Adobe Flash and your other plugins updated.
Moving to the latest version of Opera, Google Chrome, IE10, Firefox or other would be advised.
Thanks.
