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 last level with a single bullet.
Complete the Black Pawn Scenario.
Welp, there you go.
Solve Vinny's riddle on your first attempt.
Find the developer's future game concept art.
Screw around in the fireplace.
Press E on every single bloody thing.
Stop looking at the damn picture.
Complete the White Pawn Scenario.
A sizeable achievement.