Oblivion: The Gates of Hell mod for Amnesia: The Dark Descent

Here I will list up where you find the different keys and important items to progress, and how to use some of them. Here we go:

Burning house:
Stack up some boxes to get out of the house, up to the hatch in the roof. If you get high enough you will be able to climb over easily.

Small house:
There is a door behind the shelf, the key for it is in the desk.

Cellar:
There is a key in one of the rooms nearby. It has a blood trail leading to it, and a burning furnace. the key is well hidden somewhere in the room.

Cellar 2:
There is a locked door at the bottom of the stairs, therefore you must go under the stairs and find the key for the door under a bed. Under the next stair after the brute passes by there is a locked door where the brute went, and another locked door under the stairs again. You must go to the cistern to find the key to the door under the stairs. After that you must find the key for the other locked door, and you can proceed to the next area, the prison.

Prison:
Here you must find the key for the door to the right. Beware of a silent brute on the way! The key is on the far end of the hallway, through a blocked door. Use a heavy item to push through, and get the key.

Machinery, Old Bunker and Deepest Floor:
This time around there is a break from the keys! You have to fix a machine to stop gas from leaking out from pipes. To do that you must find three machine rods. One is located close by, in a room with a lot of boxes. The two others are in the Deepest Floor, and the Old Bunker.
In the bunker the key is located on the right side of the map, behind some bookshelfs. In the deepest floor you must complete another small puzzle. You must find a drill to lock up the door in front of you, which is located to the left of the map. To get up through the hole afterwards you need a ladder, which is to the right. After placing the ladder you can climb up, and get the last rod.
Now you must place all three rods in they're slots, and the gas will be gone.

Dungeon:
You need a key to open the door in the hallway. It is in one of the cells. After that you need a hammer to break through the damaged wall, behind the boxes. It is to the right through the metal door. Keep in kind that this hammer is needed later into the story, so be sure to pick it up, even if you somehow glitch your way past.

Main Hall:
You need to find the key for the kitchen, which is to the right, through one of the doors. After locking up the kitchen door you need the key for the dining hall, which is under some bread. After that you need to open the door to the East Wing. To do that you must go to the West Wing first.
Go to the West Wing, and destroy the damaged brick wall there. In the area inside the wall there is a key. This key is just a failsafe, as the monster that spawns in the area destroys the door which the key is used for. If the monster kills you it will de-spawn, but you will still have the key, unless you already used it on the door. Now you will find another locked door, which you need another key for. Believe me, I also see some repeating in this. The key is in one of the three other rooms, inside a broken drawer. Go open the door and you will find the key to the East Wing in a glass container.
Now go open the East Wing, and there you will see a hallway. Go to the last room to the left, and there is a key in the fireplace. (The note tips you about that, shouldn't be hard) then open the locked door, but watch out for the monster that spawns. In the library you can climb a book shelf. Do that and you can easily avoid the monster, plus get some planks so you'll be able to get over to the key, on top of another book shelf. Get it and open the next door. After this you'll find yet another locked door... Follow some blood tracks and you will find a bloody dagger, which you can use to open it with. Now go down the stairs behind the moveable book shelf and find the key down there, to open the door to the upper floors.
Now you should be back in the main hall, and ready to open the locked door. Go into the upper floors and you have gotten through this area of the story.

Upstairs
This is another complicated map. First, go up the stairs, all the way to the top. Then follow where the monster walked, and watch out, as he comes back from behind the shelf. Find the key behind the shelf, and you can open the door to the guest halls. The guest halls are optional, but it has one of two keys to the Top Floor. Not getting this key will result in you missing an important note.
Destroy one of the windows where the monster was, and climb up outside the windows. Go inside the floor above you before going further. Then go inside that and get the second key to the Top Floor. Now, if you have both Top Floor keys, go further up as high as you can go. Destroy the window and get inside. Now open the two doors, and the door to the left will have the key to the secondary kitchen.
Some may get stuck here. The secondary kitchen is at the bottom of the two large stairs again, which is an area you might already have explored. Go there and open the locked door, which is close to the door with two red candles. There you will get a hollow needle, so you can open the Master Bed Room door. Do that and go inside.