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.
Make enemies with all of your friends.
Jonas is your new step brother.
Keep Ren and Nona from dating.
Complete the story.
Bring Michael back from the dead.
Get Ren and Nona to date.
Find all hidden frequencies.
Collect all of Maggie's letters.
Collect 8 of Maggie's letters.
Collect 4 of Maggie's letters.
Jump the crazy chasm in the woods.
Slap Ren in the face.