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