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