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