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