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.
Complete "Hotel"
Meet the Banshee
Complete "Train"
Meet the Director
Meet the Passenger
Witness the secret ending
Complete "Pool"
Complete "Hospital"
Complete "University"
Complete "Summer Cottage"
Meet HUGE MAN
Meet the Principal
Meet the Doctor
Meet the Marionette
Meet the Stalker
Meet the Crawler
Meet the Patient
Meet the Bruxsa
Meet the Lumberjack
Meet the Nixie
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