A question.

For soloing mainly melodic minor and pentatonics. But really, they're not that theory based, they just play it so it sounds good. They've had no formal training etc.
 
a lot of there stuff does have sort of a classical style of composition with plain old minor scales, such as the beautiful acoustic solo instrumental section in the middle of a fair judgement....but they seem to bend the rules quite often. their earlier stuff seemed to follow the rules much more. some of their neatest riffs are nothing more than simple inversions...such as the opening riff to advent, or prologue....or the closing riff to april ethereal. (inversions rule) then songs such as serenity painted death at "white faced....this spd...," they don't really have a recognizable key...its just pure evil...its greatness...
 
The thing I love about Opeth, is that they show people how heavy you can actually play in standard tuning, and it's not all about tuning down to fart belching frequencies. The AFJ outro is one of the heaviest riffs I've heard, it's brilliant.