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.
Bring Michael back from the dead.
Make enemies with all of your friends.
Keep Ren and Nona from dating.
Collect all of Maggie's letters.
Collect 8 of Maggie's letters.
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 4 of Maggie's letters.
Jump the crazy chasm in the woods.