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