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.
Bring Michael back from the dead.
Keep Ren and Nona from dating.
Find all hidden frequencies.
Slap Ren in the face.
Complete the story.
Jonas is your new step brother.
Get Ren and Nona to date.
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.