Full list of all 26 I'm on Observation Duty 6 achievements.
The base game contains 20 achievements, and there is 1 DLC pack containing 6 achievements.
Witness the secret ending
Complete "University"
Meet HUGE MAN
Complete "Pool"
Meet the Stalker
Complete "Summer Cottage"
Meet the Nixie
Complete "Hospital"
Meet the Bruxsa
Complete "Hotel"
Complete "Train"
Meet the Passenger
Meet the Director
Meet the Principal
Meet the Doctor
Meet the Marionette
Meet the Crawler
Meet the Banshee
Meet the Patient
Meet the Lumberjack
Complete "Pool" on Hard difficulty
Complete "Hospital" on Hard difficulty
Complete "University" on Hard difficulty
Complete "Summer Cottage" on Hard difficulty
Complete "Train" on Hard difficulty
Complete "Hotel" on Hard difficulty