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.
Slap Ren in the face.
Complete the story.
Bring Michael back from the dead.
Get Ren and Nona to date.
Collect all of Maggie's letters.
Find all hidden frequencies.
Collect 8 of Maggie's letters.
Collect 4 of Maggie's letters.
Jump the crazy chasm in the woods.