Here is the full list of all 13 Oxenfree achievements. It takes around 12-15 hours to unlock all of the achievements on Windows.
Slap Ren in the face.
Complete the story.
Jonas is your new step brother.
Collect 4 of Maggie's letters.
Jump the crazy chasm in the woods.
Get Ren and Nona to date.
Play the whole game without saying anything.
Bring Michael back from the dead.
Collect 8 of Maggie's letters.
Collect all of Maggie's letters.
Find all hidden frequencies.
Make enemies with all of your friends.
Keep Ren and Nona from dating.