Full list of all 17 Outlast achievements worth 1,500 gamerscore. It takes around 15-20 hours to unlock all of the achievements in the base game on Xbox One.
The base game contains 11 achievements worth 1,000 Gamerscore, and there is 1 DLC pack containing 6 achievements worth 500 Gamerscore.
Finished the game without hiding under a bed or in a locker
Dropped the corpse in the elevator shaft
Collected all documents and completed all recordings
Collected 15 documents and completed 15 recordings
Finished the game
Collected the key in the Female Ward
Activated the sprinklers in the Male Ward
Drained the Sewers
Restored power to the Administrative Block
Finish the game in Insane mode
Finish the game in Insane Mode without reloading camera batteries
(DLC) Finished the DLC in Insane mode
(DLC) Completed all recordings
(DLC) Collected all documents
(DLC) Finished the Whistleblower DLC
(DLC) Turned off the electricity
(DLC) Turned off the gas