Here is the full list of all 43 Only One achievements. It takes around 0-1 hour to unlock all of the achievements on Windows.
End Game
12 Lvl completed
Music 12
Last Step
11 Lvl completed
Music 11
10 Lvl completed
Game Over
Music 10
9 Lvl completed
Music 9
8 Lvl completed
Music 8
7 Lvl completed
Skeleton 1 Dead
Skeleton 2 Dead
Music 7
6 Lvl completed
First Death
Music 6
Middle Game
5 Lvl completed
Music 5
4 Lvl completed
Music 4
1/3 of the game
3 Lvl completed
Music 3
2 Lvl completed
Pause 2
First Restart
Pause
Music 2
1 Lvl completed
Bandit 1 Dead
Bandit 2 Dead
Jump
Go Left
Fall
Go Right
Start Game
Music 1
Mute