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