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