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.
Get Ren and Nona to date.
Collect all of Maggie's letters.
Collect 8 of Maggie's letters.
Find all hidden frequencies.
Collect 4 of Maggie's letters.
Jump the crazy chasm in the woods.
Slap Ren in the face.
Play the whole game without saying anything.
Jonas is your new step brother.
Make enemies with all of your friends.
Keep Ren and Nona from dating.