So, I'm bit of a lazy guy. I just unlocked the tweaks and Old School Mode, but I'm not quite sure what exactly Old School means? Is it the only mode tweaks are enabled in? It is insanely difficult? I don't see anything in the manual describing it and my google search came up with nothing. Any ideas?
PS: Do enabling tweaks turn off achievements?

This is really odd, I've literally spent a 2 hours sorting through gaming sites like IGN to try to find the answer "What is Old School Mode?" and there is no mention of it anywhere. At all. It's very strange that this is so much of a mystery.

just beat the game a few hours ago, and then started again with the adventure+ on old school difficulty.
i dont know if its the only way to use tweaks.
the enemies seem to be alot harder to kill.

i hope this answers your question

Ehh I was hoping for something a bit more concrete.

I'm starting to believe no one really knows.

Old school mode is: -10% health, +60% enemy health, and +40% enemy damage. Those scale in the difficulties (slightly harder etc).

Wow. Where did you get these figures from?

it is a bit tougher than hardcoe, but the main difference i noticed was a lot of the times where you would have the longer "optional" side-paths. you are now required to go through them for a key to continue

You can use the perks on any difficulty, but so far as i know you cant use perks on xbox live.
Also Playing El'ara the auto aim w/o zooming does not work. So either have to zoom in or have an imaginary crosshair.

The strategy guide I downloaded off some site.

Yeah good point. In my case I completely missed out on the 3rd brother's key on my 1st playthrough because I totally skipped an area of the city of (Llyr I think it was?) and would have done so again had the game not stopped me from going through the door without a key

It's similar to Fallout's difficulty scale.

Easy = you do more damage, enemy does less
Gamer = Equal damage
hardcoe = you do less damage, enemy does more
Old School = you do even less damage, enemy damage increased further

Just use unlimited arrows, stoneskin, mana reduction and 1-hit kill. It makes Old School easier than Casual. You can even 1-hit bosses, well, at least each phase of a boss fight — cakewalk.