Here is the full list of all 10 Only If achievements. It takes around 2-3 hours to unlock all of the achievements on Windows.
Complete the White Pawn Scenario.
Complete the last level with a single bullet.
A sizeable achievement.
Solve Vinny's riddle on your first attempt.
Find the developer's future game concept art.
Complete the Black Pawn Scenario.
Welp, there you go.
Stop looking at the damn picture.
Screw around in the fireplace.
Press E on every single bloody thing.