Here is the full list of all 12 OSES achievements.
Kill 3 zombies with one shot using the sniper rifle
Kill 3 zombies in 3 seconds using the pistol
Kill a zombie from a really short distance using the shotgun.
Apply damage to 5 zombies without releasing the trigger using the laser gun.
Complete all the official map challenges.
Kill a zombie from a long distance using the assault rifle.
Accumulate 20,000 credits in the game.
Buy all of the items from the in-game shop.
Shoot 1,000 times on a single match.
Complete a classic map without killing a single zombie.
Kill 1,000 zombies
Upload a map to the workshop.