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