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