There are so many variants of Opas that the particular variants will probably be marked differently in different antivirus products. I.e. when avast! announces that Opas.P was found, it is possible that Symantec would call it Opas.N or something like that.
In any case, you should try the
avast Virus Cleaner - it detects and removes all the Opas variants that we ever got (and it is possible that the exact name of the variant will be different even from avast Home/Pro - to be able to delete all the Opas temporary files, the Virus Cleaner has to distinguish more variants than avast! itself).
If the Virus Cleaner would not detect the Opas variant you have (make sure you are not running any resident antivirus protection that would prevent it from accessing the files), it would be nice of you to send us the virus sample - so that we could add it.
Last note: to run the avast Virus Cleaner, you don't have to start your PC in the Safe mode; in fact, running it in the "normal" mode is better.