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