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