Here is the full list of all 11 OU achievements.
You found yourself in the strangest of places.
You were eaten by the Saudade Specter.
You met Gemini.
You obtained the Saudade Specter's power.
You chose a world that only has you and Gemini in it.
The world was flooded by the rain.
You made Gemini disappear.
You made a new world hatch from its shell.
You saw the true ending.
You found all the Stickies.
You realized who the real protagonist is.