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