Here is the full list of all 13 Oxenfree achievements. It takes around 12-15 hours to unlock all of the achievements on Windows.
Complete the story.
Make enemies with all of your friends.
Jonas is your new step brother.
Keep Ren and Nona from dating.
Collect 4 of Maggie's letters.
Jump the crazy chasm in the woods.
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.
Slap Ren in the face.
Get Ren and Nona to date.