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