Here is the full list of all 13 Oxenfree achievements. It takes around 12-15 hours to unlock all of the achievements on Windows.
Complete the story.
Bring Michael back from the dead.
Keep Ren and Nona from dating.
Jump the crazy chasm in the woods.
Play the whole game without saying anything.
Jonas is your new step brother.
Collect 4 of Maggie's letters.
Collect 8 of Maggie's letters.
Collect all of Maggie's letters.
Find all hidden frequencies.
Make enemies with all of your friends.
Slap Ren in the face.
Get Ren and Nona to date.